Here's a good little guide for making a small silent PC. (adjust hardware as needed to modernize)
I'd explain to your friend that you can keep it a reasonable size, but, too small and it will not only cause possible heat issues, but not be very expandable.
Something like the link below sounds like it would be a fairly small, quiet, and capable system.
*Personally, I'd just let the guy buy a gaming laptop. It sounds to me like he can't fit a decent desktop in his setup. I suspect that you're wanting to build him a system so you can build a system, not because it meets his needs.Edited by Chargeit - 3/15/14 at 10:08am