stripped seat belt bolt
I think everyone is forgetting that these bolts are inside of a plastic box that there is no possible way to get a grinder,welder,vise grips or anything into unless he destroys his seat belt mechanism in the process! all he has to work with is a a rough 1 1/2" wide hole. Which is why I suggest drilling the bolt and easy out extracting it.
also, being that these bolts are about 2" long and that about half of the length of these bolts are shanked (not threaded) he can drill in half the length of the bolt and easy out before he would even have to consider worrying about swelling the threads into the actual nut mount of the car.
worse case scenario is that even IF he were to brake the head of the bolt off (which I highly doubt....mine were grade 8's) then he could atleast remove the seat belt retracting mechanism and have better access at the bolt.
I say drill and easy out.....of course it wouldnt hurt to let it soak in some penetrating oil before hand.
good luck