What should I do!?! :( Stop being her friend or break up with him?

Okay, I have been datin this guy (who is perfect) for about 3 months. I thought everything was great until i was lookin through my bestfriend/cousins phone one day and seen that she had been callin him every day. I asked her about it and she got mad at me for askin and said "i cant talk to me friends?" So then i decided to just drop it. But one night he was over at my house and i feel asleep and my brother said him and her was like all over each other when i was asleep and she walked him to his car and didnt come back for a while! I dont jnow wat to do... Should i stop being friends with her and break up with him?