If it's the main administrator account to the computer, I'm not sure how you
can change the password if you don't know what it is. I know you can disable
the administrator account in the local policy. Probably not too much help.
If you mean that your ex-friend was using the built-in Administrator
account (very bad practice), then you cannot delete it. You can change
the password without knowing the old one by using NTpasswd to change it
to a blank.