Ah, ok. Good locking technique (around a frame tube), locking skewers/track nuts or a separate cable lock for the wheels and a quality u-lock and he should be fine.

Cheap option: move his fixed gear parts over to a vintage frame or keep breaking the same cheap fixie and having the shop replace it.

More expensive option: Soma or Surly frame but take off or tape over the decals

All that said, frames can still break, especially under big strong folks, but he would get more life out of a quality frame.