Sounds to me like he has an intermittent fault on his graphics card.Might be worth checking that it is properly seated in its AGP slot and that the cable between the card and the monitor is properly connected.
Use one of the free trial 3DMark programs to put the graphics card through its paces as it may be the demands of ther games which spark off the problem.
Might also be worth running the machine with the top off (desktop) or side panel off (tower system) to see if temperature is a factor.
Hope this helps.