If you go to that course's participants page as an admin, do you see the user listed as a teacher or participant? That user should also appear on the course front page as a teacher. If he/she is not then you have no assign him/her successfully in the course. Once you assign the user a teacher role in a course, he/she should not asked for an enrolment key. You can not tell what role this user has by looking at the roles tab under his/her user profile, you will need to look for the user's id in mdl_role_assignment table. That tab provides information of user related roles (parents, mentors) etc.