Roberts had been in Ranfurly earlier on the evening of March 5, drinking with friends and then drove to the Patearoa Hotel with three men in the vehicle, including Mr Stringer. Roberts drank beer and a whisky at the hotel and the group left about 11.30pm.
The front-seat passenger felt "uncomfortable" with Roberts' driving and fastened his seat belt. The defendant failed to negotiate a bend on the Ranfurly-Patearoa Rd, 12km east of Ranfurly, the car slid 55m along a ditch and "barrel-rolled" 64m before stopping.
Mr Stringer was not wearing a seat belt and was thrown from the vehicle. He was unable to be resuscitated at the scene.
Roberts had a blood sample taken five hours after the crash which revealed a blood alcohol level of 68mg. The level at the time of the crash was estimated to be between 126mg and 179mg, well above the limit of 80mg, Judge Michael Crosbie said.
Counsel for Roberts, Stuart Blake, said the defendant would have to "live for the rest of his life knowing that his actions led to the death of a young man, a young man he called a friend."