I'm horribly confused and I think I may not be understanding your situation. What rules did your husband's girlfriend break?
Ok, you and he had the no penetration rule (assuming vaginal intercourse) but his girlfriend is mtf so the no penetration rule wasn't applicable? (I'm assuming she is pre-operative.) Or did they have anal penetrative sex and so broke that rule? What other rules or boundaries did the two of you (your husband and you) have? Did she know those boundaries?
His girlfriend has the rule that your marriage has to be strong for her to be involved with him. That kinda makes sense - I am also hesitant to get involved with a struggling couple. But you mentioned here or in another thread that since he had sex with one of your best friends and not openly and honestly while you were away meant that the marriage was in a bit of a crisis. And then you and he covered that up in front of the girlfriend or minimized the trouble your relationship is in to her? Did I get that right? If you don't tell her you two are having issues how is it her fault she didn't follow her own rule? Maybe you don't mean it this way but to my mind that reads like you are blaming her for not being a mind reader and *knowing* that your husband and you are in marriage difficulties and stepping aside.
I also don't get why the rules your husband's girlfriend's other partner (some names would be lovely just to avoid constructions like that!) have to do with you and your husband and his girlfriend. Did she break rules her other partner set for their relationship? But more importantly why is that your problem? Here is what I mean. Don't import another couple's issues into your own relationship, and into your husband's relationship with his girlfriend. If she did break those rules, that is of concern to her partner and her - you and your husband are not automatically a party to this, even if the issue happened because of interactions with your husband. Bluntly, it's to be sorted out between the two of them, not between you, your husband, his girlfriend and her partner. Yes, sometimes a pattern emerges that someone is incapable of following any rules or boundaries. It may be that your husband's girlfriend is one of those people. Or her partners and metamours are setting impossible boundaries while communicating poorly.
And that thing about you being 'involved' in their relationship from your husband's girlfriend's partner? What the heck does that mean? You have to be there when they have sex? Threesomes only? Are you supposed to monitor them or something? That reeks of overly controlling behavior to me. It makes no sense and is a ridiculous imposition. Why is everyone all up in each other's business? You are not a quad, correct? Handle your business and let them handle theirs.
Finally, I think you are projecting much of your anger and resentment about your husband's actions onto her. He is the one who broke rules and boundaries. He had sex with one of your best friends. You don't trust him now and seem to me to be projecting that lack of trust onto his girlfriend. She isn't the cause of the problems between you two. I suggest first working with your husband - who seems willing - to rebuild trust between the two of you and don't concern yourself with his girlfriend's other relationships. Not yoru problem and you have much bigger and more important fish to fry.