What I do is duplicate the teacher role and call it something like 'Editing Support Staff'. This means that this new role has exactly the same rights as the teacher. You can then assign this role to the user.
To hide the person (actually the role), go the APPEARANCE>COURSE MANAGERS and ensure that only teacher is selected. This then allows the user with 'EDITING SUPPORT STAFF' rights to work as the teacher within the course but will be hidden from everyone except other teachers for that course.