Steve,
I'm sorry for your pain. It must be a full moon or something, because a few people have really responded to you with both barrels.
I have a few thoughts for you.
1. Your pain is real and justified. You were blindsided by bad news about someone you hoped could one day become your wife. Sure, she had backed off of your friendship, but you were still betrayed in a sense, because she always represented to you that she had high moral standards.
2. To help ease your pain maybe a little, it might help to stop thinking of her in a positive light. That was a light she cast which was an illusion. If you refocus your thoughts toward the person you now know she truly is, certainly the feeling of loss has a chance to be minimized. In other words, you didn't lose much when you lost her.
3. Being friends with her in the future. ...don't thin so. Sure, forgive her, but don't nurture her with your kindness and friendship.
4. Exposing him to his wife is tricky. Many people here see it as black and white. She deserves to know. Period. Everyone is entitled to their opinion. IMO, I believe it is the right thing to do, but not at the expense of your reputation, job, physical safety, etc. My approach might go something like this. Notify his wife via email, annonymosly, and via regular mail, anonymously. There are ways to do both very easily. Ask her to reply to you via the anonymous email (there's a way to do this) so you know she received it. In the email and letter simply say this. "I am sorry to inform you that I believe your husband is cheating on you. For proof, ask to read his text and email messages right now, before he has time to delete. I am notifying you because I believe you have a right to know."
5. Many of us here were fooled by our spouses. I would have never in a million years thought my wife would have a love affair with her boss, or anyone else for that matter. You were fooled in the sense that you misunderstood who she really is, because she lied to you. Be thankful, my friend, that she didn't take more of an interest in you. Be very thankful. THAT, is your silver lining to this cloud.
6. Finally, I hope your faith serves you well here.
Best to you.
NMAI