This sound like a replica of my situation, and I went for it, after I saw her and watch her break down and cry that she was lonesome with a hubby that doesn't seem to know she exist. I love her as friend first and she was my old flame, and let the feelings of that allow me to make the wrong move.
What I found was that she had no desire to quit the unloving relationship because of security and time invested in the platonic marriage. I also found that she wasn't the one for me, and that we had a better friendship relationship before we had an affair. I broke it off because I couldn't allow herself to live a lie, but encourage her to live the truth in being herself. I think she found someone else to have an affair with, and that maybe was the truth I wasn't willing to be a part of.
Now, I have a new girl far outstrip any notion of romance and trust that I thought could be possible. But I lost a friend that I could have shared my joy in witnessing my new union with my new love. If I could reset the button, it would be to not have that affair and stayed as friend.