I certainly would have looked at the PSU first, did you try with another?
Maybe one of the cards has a fault, try removing all the cards, even the video card, should power up, might just keep beeping but atleast it will narrow things down, if it still shuts down then I would check all connections, failing that it's possibly the motherboard. I'm not familiar with that computer including the bios arrangement, can he enter the bios, and if so does it shut down in there? if not it could simply be the battery.
