The two men and Alderson's cousin, Chayne Kaio, were in the car heading home from a party in Wainuiomata on the night of the crash.
Alderson was "significantly impaired by the combination of cannabis and alcohol" in his system, although it's unclear how much THC was in his blood. His blood alcohol level was more than double the legal limit, at 101mg.
McMenamin had about the same amount of alcohol in his system when he was tested at Lower Hutt Hospital.
A Police Crash Investigation Report concluded the car was in poor condition and should not have been driven that night.
"It had an expired warrant of fitness with a number of faults identified that would have prevented it from passing an inspection," Coroner Scott said.
"The rear tyres were found to be unsafe, having worn through and beyond the metal reinforcing belt. The front tyres were not up to warrant of fitness standard."
After hitting the bridge abutment, the car came to rest driver's side down.
Kaio's evidence was that she unbuckled Alderson from the driver's seat of the car and pulled him from the vehicle.
"She pulled him to a place of safety and remained with him although sadly he was deceased. She did not go back tot he car again to rescue Beau in the same way."
Kaio and McMenamin did not know one another and had only met at the party.
Scott believed even if McMenamin's reason for saying Alderson was driving was to protect himself, Kaio had no reason to protect McMenamin and was unlikely to have lied about it.
She died in tragic circumstances a few months after the crash, so could not attend the hearing and be questioned on her statements, Coroner Scott said.
One issue that caused the coroner "considerable difficulty" was the fact that firefighters found McMenamin "tangled" in the driver's seatbelt, with half of his body hanging out the rear window of the car.
"I questioned [a traffic engineer] carefully about this, because to my mind how this happened was crucial. [He] was quite clear. He said that it was not surprising that Beau might become caught by the driver's seatbelt while trying to get out of the car himself. The car was virtually upside down and on its side with the driver's side nearest to the ground. It was dark."
McMenamin did not remember how he came to be in the position he was in when rescuers found him.
While it cannot be proved beyond reasonable doubt who was driving - as is the threshold in a criminal court - the Coroners Court requires only that the balance of probabilities be met.
While three pathologists all indicated they couldn't establish with certainty who was driving at the time, they all agreed it was most likely Alderson was driving, particularly because the place of most risk in the car was the driver's seat.
"This is a situation where a number of young people ... journeyed around Wainuiomata in an unwarranted and dangerous vehicle, while the driver and probably all of them was seriously impaired by alcohol and or cannabis, or were drunk."
Coroner Scott said the situation was a "classic no no" and that the car's occupants had other options available to get them home, instead of being driven in an unsafe car with no sober driver.
