disk full - Mac OS won't boot

Solution 1:

You could try booting from the Snow Leopard installation media and free up space from there. It's been a little while since I've used a pre-Lion system, but IIRC, the main drive should be mounted and available from the installation environment. Just don't actually run the installation!

If your friend is having problems booting, it's probably worth checking the disk with Disk Utility as well from the installation environment before writing to it.

Solution 2:

You can start by turning on the computer and holding down the T key on the keyboard to get into Target Disk Mode, then connect via FireWire or Thunderbolt cable to another Mac which then mounts the hard drive on the desktop to enable you delete or backup files. You can free up space this way, which should then allow you to boot up normally.