Underscore character just indicates the system is live, but can't go to the next step in the boot process, which in this case would be attempting to locate a hard drive to find an operating system (or a floppy drive or CDROM, depending on how the system is configured to boot). Or as DougFrippon suggested, sometimes a bad program on a disk can cause a hang; I can't boot to IBM's Drive Fitness Test disk for some reason, it hangs the system right after loading the OS, I believe I worked out that it is the mouse driver. Just because nothing showed up on the screen doesn't mean nothing was loaded, as the OS boot can be set up not to display progress.
Does the system attempt to read the floppy drive or optical drive after POST?
Use the key combination needed to get into the BIOS setup during POST, and check the basic BIOS settings page, make sure it shows all his hard drives and optical drives.
A physical test would be to try booting with only the floppy drive connected (means he has to open the case of course), see if it says "no operating system found" or something like that. Then try with just the CDROM, then just the hard drive, eventually you might find which one is causing the hang.