How to return back to your login page when installing mountain lion?

When my sister tried to install the mountain lion developer preview 1 into the Macintosh HD Partition, while it's installing the set up and nearly finished. Instead of continuing to restart the computer, it said "about -1 minutes, -2 minutes, -3 minutes, -4 minutes, -5 minutes, -6 minutes, -7 minutes etc. She suddenly noticed that something must have gone wrong. What should she do to return back to the login page and dis-install the mountain lion developer preview 1? Please reply ASAP!!!!