"It will send a wrong signal and will also create a feeling of insecurity amongst the victim party and will adversely affect the aim of the court to conduct free and fair trial," the judge said, calling the case "sensitive" and the government lawyer's actions "professional misconduct".
Junaid Khan was killed in June at Asauti, just 30 kms from Delhi after a quarrel in a train over sharing of seats. According to the police, the teenager was allegedly abused on religious lines also before Junaid, his brother Haseeb and their two cousins were slashed with a knife before the teen was thrown off the train at a station.
During the trial, Faridabad trial judge YS Rathor spotted Haryana's Additional Advocate General Naresh Kaushik assisting the Naresh Kumar's lawyer and was told that he could not appear in court for an accused because he represented the State.
Mr Kaushik had then stated that he was only watching the proceedings, the order said.
"However, he has been assisting the defence counsel at the time of cross examination of the prosecution witnesses and suggesting questions to be put to the witnesses," judge YS Rathor said in his order the next day.
Naresh Kumar is the second senior government lawyer in Haryana to land in a controversy in the last few months.
In August, the government had to sack one of its top lawyers, Deputy Advocate General Gurdass Singh Salwara, who was caught on camera holding self-styled spiritual leader Gurmeet Ram Rahim Singh's bag after his conviction in a rape case.
When NDTV contacted Mr Kaushik, he denied the allegations.
"I have not committed any wrongdoing but was in court only to provide a photocopy of the provision in the law," he said.
His boss, Haryana's Advocate General Baldev Raj is yet to take any action. "I have not received any intimation from the court. Once I do, I will ask for Naveen Kaushik's response," he told NDTV.
Perhaps, the most famous case of such collusion between lawyers from the defence and the accused sides was in the Jessica Lal murder case. In 2008, the Delhi High Court had debarred lawyers from the two sides, RK Anand and IU Khan from practising law for four months. The next year, Supreme Court upheld the ban on Mr Anand but let off the special prosecutor Mr Khan, observing that his conduct was improper.