This might help you:
Reboot into single user mode (cnd+S). After the screen lodes, (BTW, $ takes the place of your own name) Do this:
$ fsck -y
$ mount -uw /
$ rm /var/db/.AppleSetupDone
$ reboot
You will then be in the setup assistant again on reboot. DO NOT CHOOSE THE SAME SHORT NAME AS BEFORE. The new user will be an admin. Use it to change the password of the other person, login as that person, and then delete the new user.