Hasn't he got any recovery disks? for example, if that happened to me om my Hewlett packard, I would press F10, when the blue screen logo appears, and just follow the onscreen prompts, which allow me (in my case ) to "reset" the computer to its Factory settings., or do a full format. Can't remember, only did it oncne, But I do know that I retained all my files (more or less anyway)
What else I would do is enter Bios, unselect/select the Hard drive then see if it reboots. If not, i would open the case and take the IDE wires off the Hard drive, (fiddle about with them etc.) Failing all the above I would end up doing a format. I'm really A novice, so wait till someone comes along that knows what there doing, then maybe you will get help there.