Two thoughts:
1. I don't think it is surprising that you are having this trouble. Even though you two are each in the lifestyle, it sounds as though you are not in it together. You are not functioning as a couple, you are functioning as two singles who are both in the lifestyle. Until you learn to think and act as a couple, you're gonna have these kinds of problems.
2. Her friend who helped your gf cheat is your problem, not hers. You have to make the decision about it rather than pushing the decision off on your gf. She may choose to give up this friend or she may not, but that is not your choice, it is hers. You have to make the decision about what you will tolerate in your relationship and what you will not. If you will not tolerate the "friend" being in your gf's life, than perhaps this is not really the relationship you think it is.