^ I agree with this. It's one thing to face off against the
DM's monsters, but it's another entirely when another mind is assigned to be a villain. It might raise the stakes a bit, and if this person does it well, it might just make for a memorable game.
One thing I'd be aware of though, is if he becomes too attached to the idea and wants to win against the party. If the party is supposed to win in the end, it might be a good idea to let him know that in advance, so it doesn't accidentally kill the party.
Good luck!