Boot the computer off a Slackware boot disk.
Mount the hard disk of the machine.
As root, edit /etc/shadow and /etc/passwd and remove the ecnrypted password, which is right after the "root:" entry. That should do the trick.
Important!! the format of the files is this:
name : password : info : info : info : shell
You should only remove the password, but leave everything else, including the ":" intact or you are in trouble...
Of course, I have no idea what I am talking about... So don't come to me if it does not work (or if it screws up your friend's machine)...