How to handle a toxic metamour
My husband has been in a tumultuous relationship with his girlfriend, "Mary", for almost 2 years (the entire time we've been open). Mary was a friend of ours for a couple of years before they got together and is part of our large social group. Most of the time she is fun, but I think she has PMDD because once a month, like clockwork, she goes crazy (and I say this as a person who dislikes that women get categorized as "crazy" when they are emotional).
I swear, their relationship is like a high school romance they fight so much and do the break up/make up cycle constantly. When she has these crazy periods, she is highly irrational, angry and frustrated and has unreasonable expectations of my husband. To give some examples, we go camping with our group and the first night I had to find someone else to sleep with (just sleep) because she didn't blow up her air mattress and they slept in my and my husbands tent/bed. The next day I told her she needed to blow up her mattress so they could sleep in there and I could sleep in my own bed. She didn't do it and then threw a fit at my husband because she somehow expected him to do it for her (even though she did not ask him to). I mean, a serious fit, throwing stuff and screaming at him in front of everyone. There are several more examples of her doing things like this, from being angry that when they were trying to have a quickie before he had to leave for work he set an alarm (apparently that made her feel undesired), to being pissed off that after being gone on a business trip for two weeks he spent his first night home with me instead of her.
The most recent incident was this weekend where she asked him to do something for her and then lost it when he had a question about how to do it, calling him an idiot and telling him that he was not a man. She even brought me into it saying "no wonder your wife needs someone else" (I have a boyfriend). I am LIVID. Thus far, I have done my best to stay out of their relationship and let my husband deal with it even though the things she does makes my blood boil. I tell him what I think when he asks my opinion, but then I let him do what he thinks is best - which is always to make up with her and then go on as if nothing happened. To make matters worse, she apparently doesn't feel that she needs to apologize for the things she says and does when she's in her crazy time because "that's just how she is and she's not going to apologize for being herself".
At this point, I don't know what to do. I would have broken up with her a long time ago if it were me, but it's not. I told my husband that I felt she owed me an apology for this last outburst because I feel that she disrespected me, him and our relationship, but I was told that asking for that would only make things worse when he's trying to mend it with her and that since it was directed at him, I shouldn't be that upset about it. I told him that made me feel like he is prioritizing her feelings over mine and was told that it was something said in anger and she didn't mean it and if I really felt that way I could ask for an apology but that I'd be making his life more difficult.
So, my questions are - how do I deal with this woman? Right now I'm so angry that I don't want her in my house, but I understand that may be an unreasonable request of my husband. Also, am I out of line for wanting an apology? I just don't understand why he keeps going back and am wondering whether I should pull a veto. I'm not normally in favor of vetoes but I'm starting to feel like she is seriously toxic and I don't know that I want her in my life any longer. I could use some non biased opinions here.