I trust my girl but not her study buddy.

OK I have been seeing my girl for 3 years I love her to death and trust her with all my heart but she is in college and needs help studying so she has been studying with one of her guy friends, they study at my house because I'm possessive and Monday they were siting on the floor and instead of looking in his he was looking "somewhere he shouldn't" I was so mad but kept myself from killing him but I'm a scared he might make a move on her then I might lose her and I don't want that and I don't want to tell her she can't hang out with him because then we will get into a fight and I will have to sleep in the living room, but what do I do about this?
I trust my girl but not her study buddy.
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