I agree she is still your daughter not his. You raised her and gave her your values. I am so sorry your wife cheated but the truth is the world has allot of cheaters in it. I believe God put all kinds of people on this planet because from every relationship we grow from them. Dont be scared of the choices you are faced with at this point in life. No matter what choice you make no one gets out alive anyways. You can run from this problem and make a new life and start over or you can deal with the cards in front of you and play them the best you can. I a firm believer that God wants us to play with the cards we are dealt. We were not given a fair planet. Bad things happen to good people. The question is do you have enough love in you to accept what you have been delt?
check out this book it may help you it sure helped me!
Healing Wounded Emotions: Overcoming ... - Google Book Search 