its better to own a copy of the game and a CD key. It gives you more benefits. Like without having to worry about the bugs/virus the cracks might provide, or the ability to update or play online.
If you own a hard copy of the game, you can download a crack and use it.
if he still have the CD, he wont need a crack
there are some games where you can find the CD key if you have already installed it. I just dont know how. I think its something to do with your registry.