Yes. Formatting the HD will help. It will erase all data, though. Anyway, I'm not sure why you can't boot into single user mode, you must have done it incorrectly. Not trying to sound like a jerk, just trying to help. I'm not sure your level of familiarity with Apple hardware, so you might be trying to do something like tapping on the keys. It's not like starting up into an option on a windows box, you can't tap the keys. You have to press and hold them both immediately after hitting the power button and DO NOT let go until a different screen comes up. Command and S should yield the command line. If you do not know her user name, then once in the command line, type: cd /Users
then hit return
and then type: ls
and hit return again
The output of that command should yield the directory "Shared" and then whatever the user account names are. then you want to do what i told you to do above...
Code:
mount -uw /
lauchctl load /System/Library/LaunchDaemons/com.apple.DirectoryServices.plist
dscl . -passwd /Users/username password
replacing "username" with the user account (if there is more than one, just write your new password to every account with that last command for every different user) and password with the password you want it to be. you have to put something in it, it can't just be blank. Make it "dogpoop" or whatever you want.
Then type "exit" and the machine will reboot into the regular mac OS. log in with the new admin password when you get prompted for it.
