Adam Briggs, 23, was locked up for nine years in July 2006 for his involvement in the death of 16-year-old Carolyn Lemm who was killed in a field near Rotherham.
Carolyn was shot dead by her ex-boyfriend Lee Smith, who then killed himself with the same shotgun.
Briggs had travelled with Smith to the remote spot where Miss Lemm was killed along with her 17-year-old friend Sabrina Murdoch, who he also threatened.
He was also found to have shortened the barrel of the gun which killed Carolyn.
Briggs was convicted of manslaughter in July 2006 and is currently three and a half years into his nine-year sentence.
He has since qualified for the Government's Resettlement Overnight Release scheme and is understood to have been released for a couple of nights at a time with a four-week period in between.
Photographs of Briggs out on the town with a new girlfriend have since appeared on social networking website Facebook, prompting angry responses from Miss Lemm's family.
Timothy Lemm, Miss Lemm's father, said he felt sickened that Briggs had been let out of prison.
He said: "I've seen pictures of him on the internet smiling and having a good time and it's heartbreaking.
"We will never forgive him for what he's done and for helping to take Carolyn away from us."
A spokesman for the Prison Service said the overnight release scheme was all part of protecting the public and rehabilitating offenders.
He said: "Our first priority is to protect the public, and rehabilitating offenders is a vital part of that. That approach has helped achieve a 20% fall in the frequency of adult reoffending between 2000 and 2007.
"Resettlement Overnight Release has been operating effectively for a number of years with robust eligibility criteria and rigorous risk assessment. Suitable prisoners are released for specific reasons towards the end of their sentences with measures in place to monitor them, and recall them if necessary.
"Reparative volunteer work, training, education, and maintaining family contact all contribute to resettlement and cutting crime."