So, a little background first:
I met my MOH through our respective partners, who worked together. We got on like a house on fire, and the 2 couples started hanging out on a regular basis. h2b got very close to MOH's partner, and we became, for all intents and purposes, best friends.
Then, MOH and her partner split - which was her decision. She moved into our house (while we were on holiday, she was feeding the cat, we came home and found her living in our spare room!) and stayed with us while she searched for a house share. for the next 2 years she and her partner engaged in an on-again, off-again back-and-forth, during which we were both very supportive of both them. She repeatedly asked for our advice over whether she should go back to her partner, we repeatedly counselled her. This was particularly difficult for my h2b, who was still close friends with her ex, and he put aside his personal feelings to give her heartfelt and well-considered advice.
In the end, they decided to split for good, but have stayed good friends. They both found jobs in London and are now living together as housemates down there, but have both been dating other people. We find this exceedingly strange, but don't comment - they are adults and it's their lives.
However, just before MOH moved to London (and after I had asked her to be MOH at our wedding), she told me that she had started a new relationship, and had been with him for several months - overlapping with the time that she was asking US whether or not she should get back with her ex. h2b and I were both shocked by this - not that she's in a new relationship, but that she didn't tell us there was someone else on the scene when specifically asking us for relationship advice. h2b in particular was annoyed and hurt - he's basically lost confidence in her ability to be truthful with us. Which obvs isn't good, since she's such a huge part of the wedding.
Now, her new boyfriend has split up with her. She came up at the weekend to see me, and let it slip that she was seeing him for almost the entire 2 year period that she was going back and forth with her ex. She's devastated by the split, so I didn't say anything to her - but I am incredibly hurt that she kept it all hidden from me for so long, especially as I was very supportive of her during the whole time. If I had known all this before I asked her to be MOH, I wouldn't have asked her, because to me, if someone can lie like that, then why would I want them in my wedding? But now I feel like it's too late to say anything to her, without ending the friendship entirely (which I don't want to do). To make matters worse, h2b is really angry about the whole thing and doesn't want her in the wedding now. I don't know what to do - I don't know if I'm overreacting and my expectations of our friendship are too high, or if I'm right to be feeling this way and should broach the subject with her once everything has calmed down over the split.
What would you do?