disk full - Mac OS won't boot My friend has a problem getting Snow Leopard to run because the disk is completely full. I suggested getting into command line mode (CMD+S on boot) and delete some things, but it didn't work (nothing happens) - is it because of full disk as well or because the OS is older? What can he do?
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.