Was that 'operator error' or a problem with the shotgun?
I think we all know the answer to your question: He was in a motor vehecle with a loaded shotgun, with a chambered round, and it went bang. Safety or no safety, he should not have had a round in the chamber and he should not have had his finger anywhere near the trigger unless he intended to shoot something. Seems to me that the smartest thing this guy did was to stay away from firearms for many years.
According to my brother and the truck owner, the trigger was not touched, just settling it in on the seat.