Did he say "I want to break the game" or did he say "I know how to make wizards/rogues really effective"?

Please note that the classic "Batman Wizard" doesn't break encounters by carpetbombing with Finger of Death. They do it by buffing the party and disrupting enemies, so that the rest of the party can kill them off.

I'd say let him play whatever he wants, or get a look at what he's planning before allowing it in the game. If the other players are sticking with core/no PrC, then put the same restrictions on him. Seriously, just restricting the gold/scrolls available will limit his options quite a bit, if that's what you're worried about.