My best friend has become lowlife s. Please help me. Please!?

My best friend Carmen recently started dating another one of my guy best friends Nick. Now, I was okay with it before, and I didn't mind it. Yesterday, all of us including my boyfriend Anthony went. Her mom also went because Carmen's little brother wanted to see the movie too. They sat all the way in the front, and we sat in the back. The whooole entire time, they were making out, and it was really awkward. Like, I would randomly hear Carmen say 'a little to the left, yea now your too close, okay good'. Obviously he was feeling her up, and she was guiding him around. Me and my boyfriend weren't doing any of that, and we've been dating WAY longer. I told her to stop, because her mom was gonna see her, and she said she didn't care. I feel that she doesn't care anymore. Someone in the movie theatre even said 'look at that little slut'. I told her, and she still kept going. Can someone please help me out in this situation, how can I tell her she's acting totally indecent, and how can I make her understand?