First things first. Trina is not your friend. A friend would not be telling you that this guy likes you and in the next breath making out with the guy. Something very wrong there.
Then there is Theo. Seems to me he just likes to flirt. If the girl responds, great for him. He gets to make out. If she doesn't, no big deal. He tries someone else. I get the sense he is not looking for a relationship, but rather for just a little fun. Just my sense based on his actions. If it were a girl doing this to a guy, I would call her a tease and tell the guy not to put up with it.
I guess I could call Theo a tease also. My advice would be to stop putting up with his foolishness. Tell him in no uncertain terms to keep his hands and legs and all other body parts to himself. Then just stay away from him. Let Trina have him.