She’s probably wanting you because of the excitement and drama it creates in her relationship, and she is turned on by the fact that she is possibly cheating on her bf. There are girls like this because there was some problematic manifestation from the relationship she had with her dad. For example, having a dad who wasn’t committed to the family can create this psychological inclination to pursue guys who aren’t committed to her, and find them attractive. Once she knows that her boyfriend is committed, she doesn’t find him attractive as much, on the other hand you create drama. Yet she knows the values of the relationship with her boyfriend so she is basically using you to get what she wants, excitement, and at the same time keeping her relationship.
You already know what to do, for yourself. But I would say, do you want your best friend to be in a relationship with a girl who is willing to cheat on him? I think you should come clean with your best friend, using as much euphemism so he doesn’t over react. If you have a good friendship with your best friend, he’ll thank you for your truthfulness. Even if he doesn’t believe you in the beginning, it will open his eyes to what his girlfriend is doing. Currently your best friend might be clueless, or have some suspicion but is ignoring it, thinking that he is just being paranoid. If you come clean with your best friend, he will eventually make the decision of keeping the relationship or letting it go. Your best friend could get mad at you for making out with her. And you should apologize for that… and at the same time be willing to give him space. If he is smart enough, he’ll realize that this girl isn’t worth pursuing and break up with her.
Honestly, for some people it is hard to stop the urge to cheat on a boyfriend/ girlfriend if they are brought up in an environment thinking that it is okay. Things like this always sabotage a relationship, and she wont’ learn to stop until she has some experience ruining her relationship… which is sad.