It's possible that their computer is frozen enough to need this but it's more probable that this person hasn't figured out how to disable DontZap in newer versions of xorg. You need to disable it in your xorg.conf in order to have ctrl-alt-bksp work again:
That was one of my favorite Magic SysRq keys. It's the Linux analog to Windows's Ctrl-Alt-Del.
It's the "Secure Access Key" (SAK): You press that key and it kills all programs hooked to the TTY (incl. X, in your case) and displays a proper login prompt so that you can know what you're about to login to was run by the system and not a clever malware trying to steal your password.
