The same thing kind of happened to me. The girl my boyfriend cheated on me with had to train me at my new job. When I found out, and confronted her, she was totally disgusted with him and embarassed of herself even though she didn't really do anything wrong.
She and I ended up going shopping together pretty often.
Moral of the story is:
The "other woman" often is pretty much blameless and can easily be a great person, don't villainize her :)