Well if he wants a six-pack, getting his body-fat down is more important... I can tell you from my own personal experience of dropping a lot of fat, my stomach feels waaaaay more solid and I work my abs maybe once every two weeks. It's all the running I've been doing, in my opinion.

I have a friend who's about 5'9, 130-ish, and he has great definition in the abs... does he work his abs religiously? No, I've never seen him do any other exercise than run when he played soccer. He's not too big and burly either, though.