So, I was in your daughter's position. My parents divorced when I was 12, and I found out my dad cheated on my mom thanks to how easy butt dialing was back before cell phones had number pad locks. My dad and I didn't have much of a relationship to speak of until I was 18, mostly because of how angry I was that he ruined our family and his refusal to face the fallout he caused head-on.

I'm now 27, and we've done a lot of healing over the past decade. My dad is still a crappy husband (he's remarried)-- his email was identified in the Ashley Madison leak, and in the past year he's at the very least had one emotional affair that my sisters and I know of. But my sisters and I have come to accept that him being a shitty husband doesn't necessarily mean he's a bad father. We've had to restructure our relationship on a different understanding of him that included understanding and accepting the limitations he has as a person.

I tell you this to say that it's not necessarily a bad thing that she knows. She likely would have eventually figured it out, and she would have had to work through those feelings of anger and betrayal eventually. The thing that helped me the most was knowing that my mom had made her peace with the situation and had moved on. I would talk to your daughter frankly and tell her that yes, you felt hurt at the time, and that it's okay for her to feel angry about the situation too. But that her mom's shortcomings as a wife don't mean that she has shortcomings as a mother, and that you still respect her mother as your co-parent. She's going to gauge the way she processes this based on how you respond to her mother, so be respectful and complimentary.