If he just wants a gaming machine, and mining with a single card, then yea he can use a standard atx case with at least 400W supply and just get a single 7950, but this is not going to give room to expand.
It would be better to not buy a prebuilt machine (they come with [beep]ty psus and pc cases that don't work for mining), wasted money. He needs to invest in a good a power supply first like a lepa 1600 gold rating, or some other 1200W or higher supply. Then find a motherboard with at least 4 or 5 pci-express or pci-1x slots. And powered risers to get the cards spread out from each other. Then either design and build your own case or rig something up without a case. GPU miners generate a ton of heat. A couple 120mm fans are not going to cut it.