disk full - Mac OS won't boot
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.