Knoppix is a live CD, it can be installed permanently but is definitely not recommended for beginners.
Ctrl-alt-escape brings up xkill which can kill a non-responsive window or app, running under X windows. It wont do any permanent damage to an operating system.

No-one can help you unless you post the error messages, but unless your friend is an experienced user, he will not be able to fix the problem, which is why knoppix is not recommended for beginners or for a permanent install.

Failed to boot? You are seeing "a ton of errors" so do you mean that you cannot get to the kdm login screen or get to a console? If you are at a console, the last message maybe login, if so, the X server is not starting and the clue will be in /var/log/Xorg.0.log