Umm, this is wierd. And I don't know if this is exactly right...
What do you see in the server-manager panel?
If the user in question isn't showing, you might look in the /etc/passwd and /etc/groups files to see if the user in question exists. If so, you might try editing the files to remove the lines SPECIFIC to the user in question.
Back them up first. (login as root, 'cp /etc/passwd /etc/passwd.bak' , etc.)
G