Oh goodness. I am so, so sorry you've ended up in this situation. My gut reaction is: "In all his reading, did he forget the ETHICAL part of ethical non-monogamy??" He wasn't up front with you about his initial practice of the theories you two were discussing, and he seems to be conveniently forgetting the boundaries when he wants something. That's not how poly (or any relationship) works.
I would suggest getting all three of you together to put the boundaries/agreements IN WRITING so no one can later say, "Oh, I forgot". This would also give you a chance (if you want it) to reassure her that you think she's lovely and have no problem with her personally. SHE is not the one coming between you, it is really him and his actions and lack of respect for you.
It's possible that you'd feel more comfortable hammering out that agreement with just him first, before getting her take on it, but I'd worry that he'd change his tune between meetings. You've done amazing so far, so if you think you can handle it I really think making him communicate with both of you at once, in person, is the safest way to go. She might even have something to say to him about the way he's treating you if he continues to disregard your feelings.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack