Echoing the sentiments of others here, from experience:
DO NOT BUILD for somebody else unless you are prepared to tell them, as others have mentioned, that if the thing hoses you're just going to wipe and restore and bye bye to all the precious files they messed up.
DO NOT BUILD for somebody else if you're going to buy all your parts from Fry's, they'll get a better deal from a sale from an OEM.
DO NOT BUILD for somebody if it's a money issue, if you do then they will think you are tech support. Do it for beer and a pizza, as well as some favors, and he won't be all over you as much when he breaks it. [I assume you're not taking off 2 days of work to build this dude's computer; that's way too much time.]
If these dissuade you: Point him towards Dell, Alienware, et al, and when something breaks he can yell at some poor tech guy in India.
Last alternative: If he's a hardcore, and wants a great rig, why not point him in the right direction for some education (AT, Newegg), help him out a little bit along the way and just assist once he's ready to put it together himself. You'll still get the beer, pizza, and he'll know where to go (Forums instead of your door) when stuff breaks.