Could be a few different possibilities. Software is always to be hoped fore in these situations, because reloading is annoying but essentially free...
If it was working and just started not working suddenly, chances are its either something that got corrupted, a patch that the video drivers your friend has installed doesn't like, or hardware.
To eliminate the hardware portions from suspicion Download a copy of the free speed fan application http://www.almico.com/speedfan.php
and run the S.M.A.R.T error check, and the temperature check. The S.M.A.R.T error check will indicate if there are any mechanical issues with the hard drive. If the hard drive itself is close to 5 years old, I'd consider it suspect by default. If its only a few years old it should be ok.
The temperatures to look at will be the processor temperature and the video card temperatures. Most modern video cards are designed to operate at a higher temperature than the processor, but it really depends on the make and model of the CPU and GPU as to what that temperature range is. Get those numbers, post back here, along with what CPU and what video card he has.
Computer RAM also goes bad from time to time. that can be a pain in the **** to diagnose. Your best bet is to download the memtest ISO file, burn it to a CD and boot to that and run the memory scan, or use the windows 7 built in memory scan on the windows 7 DVD. Another alternative if your friend is running 64 bit windows 7 by any chance and doesn't already ahve alot of ram would be just upgrading the ram, as ram is presently dirt cheap.
Even if the video card temperature checks out, the ram onboard the video card can fail. Its very hard to check for that short of putting in a new card and seeing if the problem persists.
If the machine boots, it probably isn't the CPU or the power supply. If it operates normally other than gaming, it probably isn't the motherboard, though it is possible that a PCI express slot is going bad. I've only ever seen that once though.
Before we start poking around at software as a potential culprit, what version of windows, what video card, what driver revision, etc is being run here? Also is there any entries under event manager when things fail? (Control panel: administrative tools: event manager)