Should I tell my partner she can no longer be friends with her ex?
Why would you tell her that so I can better understand where you are coming from?
I asked her if they are having sex, she said no but I know she is lying.
I see your point, but it's rude to try and control your gf. You should more trusting of her.
They have sex every time when they meet.
Ouch, that is terrible. Why not just break up with her because she cheated on you? You deserve way better than having to put up with a cheating girlfriend.