You're right, I probably am being a little too harsh on her. She did mention during one of the discussions I prompted that while she was unsure about it in the beginning she thought she could handle it and work through it, which is why she didn't bring it up. Unfortunately now I am hurting and questioning a lot of the things she's told me. I also feel like a bit of a jerk for not understanding sooner how much she was struggling and continuing on with this relationship with Z that I could've just stopped and not pursued at all if I'd seen it.
Ground rules and boundaries are probably in order no matter what happens; because of our social groups and work situations we're all going to have to see each other at various points, and regardless of whatever else happens I'd like to at least continue to be friends with both of them.