For a while now he's been cheating on her with me. She lived in another city, but she recently moved to be with him. We still see each other. She's reallllllly nice, and doesn't deserve it. Neither do I. I'm not interested in being with him emotionally, or stealing him away... But I feel like she needs to know. I don't see him ever coming clean. I feel like it is partially my business, but should I just let her find out naturally? It's not fair for her to think she has such a great catch.

First of all, what you are doing as wrong, very wrong! Would you like to be in her situation one day, being with a man that cheats on YOU? Have you no moral values, how were you raised? You know that you can be prosecuted by her for braking up her marriage. My opinion is that you and the guy should sit down with this unfortunate woman, discuss it with her and take it from there.

If they are together and he was with her first then do the decent thing and back off. At least give their relationship a chance to see where it is going.