My friend is becoming a slut(and basically mean), and it's obvious. I don't know what to do!!?

Ok, so awhile ago, I saw this girl we'll call "Gina". Gina and this guy "Nathan" were going out. Well, on the way home from "Melissa"'s bat mitzvah, we were on the party bus. I was sitting with my friend "Sarah". Sarah suddenly taps me and points over to where Gina and Nathan are. Nathan has his hand up Gina's skirt, and she's laughing. Then Gina kept sticking her bracelet under her butt and Nathan would get it. And they were kissing a bunch while doing so. Honestly, I get the whole kissing thing, but we're in 7th grade, so not sure about him touching her. They broke up a little while ago, though. Then, Nathan asked out one of my best friends ever at some party, and she said yes. My freind will be "Krista". So Krista got mad at my other friend, "Nicholas", for something SO STUPID!!! Nathan hates guys in theater, but not girls, and Krista is actually in drama also. Nathan called Nicholas a gay ******, and Nicholas reported it and got Nathan suspended. Nathan also got grounded and couldn't go to the movies with Krista that weekend, so Krista got mad at Nicholas. Insane? So then my friend "Jennifer" asked Krista if she remembered what Nathan did with Gina, and she said yea. Jennifer asked Krista if she wanted that to happen to her or something, and she said, "Well... maybe". During lunch, she begged me to come with her to sit at the table Nathan sits at, which you could say are kind of normalish popularish people. I did, because she really wanted me to, so I came with her. While we were there, Nathan started talking about Nicholas, how he was totally gay and stood really weird and stuff like that.(I know he's not, he's had a girlfriend, and has asked me out before.) Now, after lunch today, they kissed! It was Krista's first kiss. Not that bad, no. But she used tongue!! It was her first kiss!! I'm not sure if this is common or what, I don't want anything bad to happen to her, and I really don't want to lose my best friend. Please help!!!!!