What happens if he doesn't like HL2 when he's played the demo?
He should wait for the full version and give that a try. I'm sure he could find a friend who'd be willing to let him play it, rather than spending $50 on a game he ends up not liking. Demo's aren't always a 100% reflection of the final product.
He could always sell it to some poor sucker on Ebay.
that is like asking what happens if he discovers he doesn't like to breathe
I think that's what a lot of people were saying about Daikatana...