At first, it wasn't clear what would happen next. Would she follow him? Or would they end up divorced?
The answer: neither. "After a few months," Mr Frost said, "we both realized we liked it this way."
Technically, the two are married. They file joint tax returns; she's covered by his insurance. But they see each other just several times a year. "Since separating we get along better than we ever have," he said. "It's kind of nice."
And at 58, he sees no reason to divorce. Their children have grown and left home. He asked himself: Why bring in a bunch of lawyers? Why create rancor when there's nowhere to go but down?
"To tie a bow around it would only make it uglier," Mr Frost said. "When people ask about my relationship status, I usually just say: 'It's complicated. I like my wife, I just can't live with her."
The term "trial separation" conjures a swift purgatory, something ducked into regretfully and escaped from with due speed, even if into that most conclusive of relationships, divorce. We understand the expeditious voyage from separation to divorce, the desire for a clear-cut ending that makes way for a clear-cut beginning. We hardly look askance at the miserably married or the exes who hurl epithets in divorce court.
But couples who stubbornly remain separated, sometimes for years? That leaves us dumbfounded. "I see it all the time," said Lynne Gold-Bikin, a divorce lawyer in Norristown, Pa, who is the chairman of the family law department at Weber Gallagher. She can cite a docket of cases of endless separation.
With one couple separated since 1989, the wife's perspective was, "We still get invited as Mr and Mrs, we go to functions together, he still sends me cards," Ms Gold-Bikin said. As for the husband, "He cared for her, he just didn't want to live with her."
But at his girlfriend's urging, he finally initiated divorce proceedings. Then he became ill and she began taking over his finances - a bit too wife like for him. "He said, enough of this, there's no reason to get divorced," Ms Gold-Bikin recalled.
Among those who seem to have reached a similar conclusion is Warren Buffett, the wealthy chairman of Berkshire Hathaway. Mr Buffett separated from his wife, Susan, in 1977 but remained married to her until her death in 2004. All the while, he lived with Astrid Menks; they married in 2006. The threesome remained close, even sending out holiday cards signed, "Warren, Susan and Astrid."
Also in the ranks of the un-divorced: the artist Willem de Kooning had been separated from his wife for 34 years when she died in 1989. Jann and Jane Wenner separated in 1995 after 28 years but are still married, despite Mr Wenner's romantic relationship with a man.
Society is full of whispered scenarios in which spouses live apart, in different homes or in the same mega-apartment in order to silence gossip, avoid ugly divorce battles and maintain the status quo, however uneasy. In certain cases, the world assumes a couple is divorced and never learns otherwise until an obituary puts the record straight.
Separations are usually de facto, rarely pounded out in a contract, and family law is different state to state. But even long-estranged couples are irrefutably bound by contractual links on issues like taxes, pensions, Social Security and health care.
Divorce lawyers and marriage therapists say that for most couples, the motivation to remain married is financial. According to federal law, an ex qualifies for a share of a spouse's Social Security payment if the marriage lasts a decade. In the case of more amicable divorces, financial advisers and lawyers may urge a couple who have been married eight years to wait until the dependent spouse qualifies.
For others, a separation agreement may be negotiated so that a spouse keeps the other's insurance until he or she is old enough for Medicare. If one person has an existing condition, obtaining affordable health care coverage is often difficult or impossible. The recession, with its real estate lows and health care expense highs, adds incentives to separate indefinitely.
Four years ago, Peggy Sanchez, 50, a Midwest resident, parted amicably from her husband, who has fibromyalgia.
"He would not get medical treatment if he weren't on my insurance," she said, and giving him that is less expensive than paying alimony. "Besides, I care about him and want to make sure he gets the medical help he needs," she said.
There are still sticky issues: Ms Sanchez's boyfriend is unaware that she's still married. Her daughter from a previous marriage views her husband as a father figure. And he got custody of the family dog. But Ms. Sanchez plans to stay separated.
"I don't have much desire to remarry so there's no benefit to me from divorce," she said. "I guess that sounds pretty jaded, but it's just not as important as it used to be."
Sharon O'Neill, a marriage therapist in Mount Kisco, NY, has seen four cases in the last two years in which couples separated but stayed in the same home. In a depressed market, couples may not want to sell a house they purchased at the market's height, or one party can't maintain the mortgage or the other can't afford a new home.
"The financial collapse has made people say, 'Let's not rush into a divorce, let's see if we can make something else work,' " Ms O'Neill said.
The added value of marriage is also hard to kick.
"Many people I've worked with over time enjoy the benefits of being married: the financial perks, the tax breaks, the health care coverage," said Toni Coleman, a couples therapist in McLean, Va "They maintain a friendship, they co-parent their kids, they may do things socially together. Sometimes they're part of a political couple in Washington or have prominent corporate positions. But they just feel they can't live together."
What Ms. Coleman finds surprising is that the primary consideration is practical and financial, not familial. The effect of endless separations on the children rarely seems a priority.
"People split up and have these God-awful joint custody arrangements, so you would think that they stay separated for the kids' sake, but I'm not seeing that," she said. "It usually comes down to money."
Others believe separation is easier on the children than is divorce. A 48-year-old social worker from Brooklyn, separated eight years, traded places with her husband in the same home, so that their children would not have to shuttle from one home to the other. The couple had an apartment where each would live when not at the family home.
"In hindsight, it was probably more confusing for the kids," she said. "But we did it with their best interests in mind."
But long-term separation can create big problems. If a couple isn't divorced, their lives are still legally and financially intertwined. If your estranged husband goes on a spending spree, you're responsible for the ensuing credit card debt. If you win the lottery, that's community property. Finances can swing wildly, creating an alimony boon or a bombshell should one partner eventually want a divorce.
"I just had a situation where after 15 years of separation, the wife wanted to remarry," said Elizabeth Lindsey, an Atlanta divorce lawyer. "But over the years, his assets had completely dissipated." The wife would have profited from divorcing earlier.
A separation can also go on longer than anyone anticipated, even until death, leaving a mess for survivors. In New York State, for example, a spouse, even if separated, is entitled to a third of the partner's estate.
There's also the risk that you could lose track of your erstwhile partner altogether. "We see cases, usually with foreign nationals, where the husband goes back to the Philippines, and the wife wants to marry James but she's still married to Ted," said Steve Mindel, a managing partner at the Los Angeles law firm Feinberg Mindel Brandt & Klein. Judges now often require that a professional be hired to locate the spouse, to facilitate the divorce.
But more often than not, a delayed divorce simply reflects inertia. Celeste Liversidge, a divorce lawyer in Los Angeles, most frequently sees people who are avoiding an unpleasant task.
"It's often so ugly," she said. "People get to a point where they can't live with each other but going through the divorce process is too painful." A six-month separation turns into years.
One woman, a 39-year-old mother of two from Brooklyn, who like many interviewed for this article wished to remain anonymous, has stayed separated for nearly two years at the suggestion of five lawyers.
"There's no advantage to getting divorced," she said. Both she and her husband are in new relationships. Most people assume they've officially split. But given the health insurance issue and the prospect of legal fees, she said, "I feel like we could just drift on like this for years."
Not being divorced is also an excuse not to remarry.
"In my day, we'd refer to a man as a bon vivant, a gadabout who doesn't want to worry about marrying anyone else because he's already married," said Sheila Riesel, a New York divorce lawyer for more than three decades.
In the end, some people just don't want to divorce. Perhaps one spouse desires it and the other drags his or her feet. Sometimes, people are just confused; separation can be a wake-up call.
In other cases, initiating divorce ultimately serves that purpose. Last year, a 67-year-old professor in New York filed for divorce from the man she married in 1969 and separated from in 1988 after she had an affair with a woman.
"I had images of Vita Sackville-West, but it was very messy and the children suffered a lot," she recalled. "My husband had been more attached to me than I thought."
And she considered him a pal; they even took vacations together. "I think I liked that we were still married in some way," she admitted. "But last year I met someone who minds that I'm still married to someone else."
And thus, time to divorce. Call it an old-fashioned romance.