What kind of person was your friend before you found out about this? Has she been a true-blue friend to you? If this affair weren't in play, would you still consider her to be a good person?
I agree with Jennifer C. Unless your friend was already a people-user or had other concerning personality traits, there may be more stuff going on than what's on the surface. Most people who are happy in their relationships don't cheat on their significant others. You say that her marriage is none of your business, but she did open up to you, and I imagine that she might appreciate some support or neutral advice ( as in "Have you talked to a marriage counselor?").
I am not condoning what she did. That said, I know that I've done things which could have alienated my friends in the past, and they could've dumped me, but they didn't. They knew I was going through a hard time and loved me without congratulating me for my failures. That, to me, is pretty damn unconditional love. I've extended that same "wow, I'm going to have to watch you so something foolish.... and I love you and I'll be here when you come back to your senses" friendship. In some ways, it's these friendships that feel the most real, because when we fall, we still accept each other.
Only you can decide for yourself which path you want to take. Granted, there are likely things which I would dump a friend for, but not having universally compatible values isn't one of them. You aren't patting her on the back and saying "Way to have an affair!", you are putting your arm around her and saying "I'm here for you. Share your heart with me."
ETA: I just read your SWH, and it sounds like you already know what you feel like you want to do. Whatever is healthiest for you is best....