I guess to put it more bluntly as many might be thinking.... I would RUN and run fast, before your heart has a chance to make a bad decision.
I lose count of the ways I would never have a fuzzy feeling for life after that ordeal. My family would pretty much shun her anyway so that would all be friction.
I also have very little confidence in ANYONE that goes crawling back to their H ONLY after getting knocked up and the boy toy runs away. I also have ZERO respect fro the boy toy either. He knew you, knew she was married, did it anyway.
I would change the locks and sever that deal.
Regarding faith. I am catholic. I know what Jesus would do BUT forgiveness is one thing. Being married to a cheater is another. You can forgive but you will never forget.