If he wants to use anything larger, it needs to be blocked so it can only hold 5 rounds plus 1 in the chamber. If you can push a 6th round into the mag it will be trouble.
best solution is to buy a factory 5 round mag. If he doesn't and he gets checked it will be a nice fine and points against his hunting license. Not something I'd mess with over a $25 mag available at most big box stores.