Can I tell my roommate that he can't have an interview in the living room?
Maybe. Why can't your roommate have an interview in the living room?
Because I want him to interview in my room.
Well, if you are generous enough to offer the roommate your room instead then they should take the offer. I see no issues with it. The living room might be too personal for interviews and show too much.
Screw you bitch, I'm going to tell my roommate that he can't do his interview at the living room
Its okay to try to help the room mate and maybe make it even better for the room mate by choosing a room that looks better for the interview.