I would say it is entirely your mates fault. The only redress would be if the car should not have been on the track at all. Even then I suspect your mate would still be apportioned a fair amount of the blame - but he may get something.
If it's acknowledged and accepted that vehicles may enter the track in the way that one did, then he should have been looking where he was going, and stopping to check his gears if it requires so much attention that he couldn't do that.