It sounds like his six point was not fastened correctly and/or his seating angle was wrong. In a proper six point, the belts are snug against the thighs, not riding up the middle against your crotch, and if his seat was at the proper angle, he would not have been thrown into the anti-submarine belts, his ass would have been thrown forward into the seat that was already there while the lap and shoulder restraints would have kept him against that seat, with the anti-sub belt just ensuring the lap belt doesn't move out of place.

Simpson and Schroth both have great documentation on the proper angles for the driver and the belt anchor points:

Safety - Restraints - Seating Positions | Simpson Racing

http://www.schrothracing.com/docs/Co...structions.pdf

There is a very low tolerance to get it right and many teams/builders just bolt the harnesses in like oh, hey I have a harness, I'm safe now. Some of the belts only have a tolerance of ten degrees or less deviation from the ideal angles, and if you deviate, you get your nuts crunched. I wear a six point in my car.