There's this girl and she's having with my best friends boyfriend?!?

A broken heart is devastating at any age but especially when you are only 13. All you can do for your friend is be there for her, give her a shoulder to cry on and a ear to listen. I know she lost her virginity to this guy and it sounds like to me that is all he wanted, she will in time get over that. She needs to never see this guy again and as far as you disliking Isabel, that is a perfectly normal feeling but she didnt do this alone, it takes two, so I think it would be beneath you to say anything to her, you know she is not a nice person and the best thing for you and Becky to do is both move on from this.