Subjecting an old low-end bike to the rigors of fixed gear pedaling and "braking" via a big, tall, strong guy is certainly a recipe for destruction. My vote is also go higher up the food chain. Or make it a single speed with a free wheel and employ brake calipers. That should make the stresses of the frame drop considerably as it will be ridden in a much more traditional manner.

The old Cannondale SR or ST frames sound good to me. They're welded, super beefy, and have fragile paint which is a great theft deterrent as he can find one that's a bit rough and not have to worry about it. Or he can get a welded steel frame with thick tubing as weight doesn't matter here.

Could he try and be kinder/gentler/smoother in his riding?