Anyone who has permission to edit the course settings (including Teacher) has permission to change the course specific role names as it is in course settings. Tell him/her not to make those changes and explain why. If he/she refuses to comply, remove his/her permission. In my experience, the people who change the role names only make themselves look silly. I had one guy who kept changing the Teacher role name to his name and then he would complain that course would say Joe Schmo: Joe Schmo.
I could be wrong, but I haven't seen any way to specifically prohibit role renaming...this is part of the Update Course Settings permission. You could remove that permission for manager, but, of course, that removes the rest of the course settings from editing as well.