Seeing as how this case was about who was responsible for the passengers death, let's look at how it could have been 100% avoided.
1st the passenger could have refused to ride.
2nd the driver could have refused to drive.
Both people had the power to avoid the passengers death. Both voluntarily acted in a way that resulted in the death.
It's very common these days to hear that "YOU are ultimately responsible for your own safety", and unless threatened or somehow forced into doing something against your will, I tend to agree.
Other questions arise like, was alcohol really THE reason for the accident? More than likely it was, but hard to prove as non intoxicated people crash and die every day in single vehicle motorcycle accidents.
Even assigning 100% responsibility to the driver for the crash is an unknown.
How do you know that the passenger didn't contribute to the crash like slapping the driver in the back of the head, screaming/yelling, punching him in the ribs, or otherwise distracting him, or moving around causing the bike to go offline, etc. Unless there are witnesses to indicate otherwise, you just don't know.
Now that we know both had a share of responsibility in the matter, the only question left is how much is assigned to each one (if in fact that's even applicable).