The normal practice would be to give the intern an account and password to log onto. The account would be the same account the files are stored under unless the files are on an external drive that is set to ignore permissions and then it could be any account. That way she appears as a normal user on the host system and would have all the privileges of that particular user account.
Just a thought, I know you do not want to invest much, or any, more money into this system, but given what you are trying to do you might consider a
Mac Mini Server that comes with Snow Leopard Server installed. It would take a bit more to setup initially, but it would neatly solve all of these permission issues you are having with co-workers, interns,
etc. The server software is specifically designed for the task you have in mind, and in the long run could make your life much easier.
