Carolyn Lemm, of Gleadless, Sheffield, was shot dead by her jilted former lover Lee Smith after he drove her to a remote spot close to a lovers' lane by the village of Woodsetts last December.
He then turned the gun on himself.
Today his friend - 20-year-old Adam Briggs - is facing a lengthy jail term after being found guilty of manslaughter and shortening the barrel of a gun in preparation for the killing.
He denied the charge but was found guilty by a unanimous jury after six hours of deliberation. He was found not guilty of a charge of murder. Sheffield Crown Court heard he helped Smith, also 20, of Lincoln, find and then shorten the gun and was convicted on the "joint enterprise" basis that he was involved in planning the murder.
After the case Carolyn's dad Timothy Lemm said: "We, as a family, are delighted that justice has been done but it is not the verdict we wanted.
"We have no forgiveness whatsoever for Lee Smith or Adam Briggs. Carolyn was only 16 years old and they had no right or reason to do what they did.
"Only families who have been through such a tragedy can understand what we are going through."
Carolyn's grandma Mary Thwaites added: "Briggs could have saved her, but he did nothing. No jail sentence will make up for his part in her death.
"He bears a terrible responsibility for what happened.
"She was a wonderful girl. She had her moments but everyone thought she was great. She was always smiling."
Det Supt Carl Sturgess, of South Yorkshire Police, said after the conviction it was the first case of joint enterprise manslaughter he had dealt with.
He said: "This was unusual in that the man convicted never pulled the trigger. But this was a wicked crime and he has shown no remorse. Smith may have shot Carolyn but Briggs played a part in what happened."
The court heard Smith drove Briggs, Carolyn, 16, and her friend Sabrina Murdoch, 17, to the field after telling them £4,000 was buried there.
Smith, the son of a wealthy businessman from Lincolnshire, ordered the girls out of the car at gunpoint, told them to drop their phones and marched them into the field.
Briggs allegedly told Sabrina to "run for her life or he would use the gun after Smith had used it".
Moments earlier, as the pair of them cowered at gunpoint, Sabrina had whispered: "Carolyn, we're gonna die" and Miss Lemm had replied: "I know."