First, what symptoms are there. Doesn't it boot at all, do you get the gray screen with the blinking question mark, do you get to the blue screen ( 😱 ), or maybe all the way to the Finder? Is he able to boot into single user mode (holding down cmd-S during boot)?
The problem might be one (ore more) corrupted preference files, and deleting some of those may solve the problem, but we need a bit more information of symptoms and which system the machine is (supposed to be) running...
If it still doesn't want to boot, you might see if the machine is able to boot from an installation disk (insert disk and hold down C on boot) he has the option of am Archive and Install, which should give him a brand new system without messing (too much) with his files, settings etc. The previous (corrupt) system will be saved to a designated folder, where he can recover things that the new system didn't install...
Edit: Backing up first, using the Firewire Target mode "tricks" that croshtique described might be wise no matter what, as it's always a good idea to have backup before system (or any large) installs... 😉