Taine Rupena Tata Bryn Edwards (22) was found guilty after a one-week jury trial before Justice Cameron Mander in the High Court at Invercargill in relation to the crash, which took the life of Invercargill nurse Emma Bagley.
He was the passenger in the car.
The jury deliberated for three and a-half hours before it returned a unanimous guilty verdict on the charge of being party to manslaughter by inciting and encouraging the driver to operate a vehicle recklessly, thereby causing the death of Mrs Bagley.
Edwards was also found guilty on three counts of inciting and encouraging the driver to operate a vehicle recklessly, thereby causing injury.
His sentencing is set for November 25.
On December 7, 2018, Mrs Bagley died when the car she was in with her husband, Leonard, and their two children, Eva and Flynn, was T-boned by a car driven by Dejay Rawiri Kane.
Kane was convicted last year for driving at an estimated speed of 124kmh with a blood-alcohol level three times the legal limit.
Families of the victim and the defendant were in the public gallery and both were emotional when they heard the verdict.
Mrs Bagley’s family declined to comment on the verdict.
The trial started on September 2, and the jury heard evidence from 26 witnesses.
Crown prosecutor Mary-Jane Thomas said in her closing address early this week that Edwards’ presence and his remaining in the vehicle had encouraged the driver.
"The Crown says Emma Bagley would be alive but for the actions of the defendant on December 7.
"This defendant wasn’t an uninterested spectator, sitting in the vehicle ... he was having a ball, he was enjoying himself."
However, defence counsel Fiona Guy Kidd QC argued the Crown had failed "by a wide margin" to prove Edwards’ liability, saying he never encouraged the driver.
"It doesn’t make sense ... that the front-seat passenger would intentionally do this given it would put his own life and others in danger."
In his evidence, Edwards told the court he "blacked out" for most of the night due to alcohol consumption.
