Whoah! OK, the background story changes everything for me. Let me just say that your friend, while I'm sure she has some wonderful qualities, needs a serious re-think about her actions. She sounds like an either really immature or very unhealthy ENFP.
Some immature ENFPs can flit around from experience to experience... or in this case lover to lover. It's sort of an extension of the being distracted by something new and shiny thing.
But in my opinion, only the most out of balance ENFPs would be so focused on experiencing new things that they would forget to take into account the feelings of the people around them.
I find it disgusting that your friend would put you in such a horrible position as being stranded in a strange place with people you didn't know that well... after treating you with such disrespect as to be physically intimate with someone else 1) after you made your boundaries so clear, and 2) when she knew you were nearby and very well might catch her inflagranti delicto.
So I would say to you, MDP2525, that you should control the only thing you can, which is yourself. In other words, set good boundaries with her and don't take any of her crap.
And, don't doubt yourself so much... if you are doubting yourself. Listen to your emotional instincts. They seem to be telling you a lot...
My ISTP is such an awesome, kind, and considerate person. But he has very little trust in his emotions. He's always analyzing them to see if he is "feeling" the correct thing. Sometimes he does this to the extent that he discounts his ability to make emotionally based decisions... to the point of out-of-balance insecurity.
I don't know if this dynamic is going on with you, but I encourage you to have faith in your feelings (good or bad) about people. Trust your intuition about this girl and whether or not her behavior is OK...
For example, your ENFP friend probably has many wonderful qualities, but that doesn't change the fact that she also engaged in some inconsiderate, nay selfish, behaviors. I'm referring to the cheating and the possessive behavior. She pushed someone away from you that you were dancing with? That is totally unacceptable.
So let me repeat again, you are the only thing you can control. Set good and firm boundaries with her. And if she can't honor them, kick her to the curb. You do neither you nor her any service by allowing her to behave in such an unacceptable manner.