I know all too well wanting to protect your child from the hurtful actions of another parent. H is his own person and if he is going to have any meaningful R with your D, then it has to come from him. In fact it is perfectly normal for her to have her own hurt and anger towards him now as well. Those feelings need to be acknowledged and honored. Do not worry abut their longer term R, that will sort out over time.
I made the mistake of not honoring one of my Ds feelings of anger towards H and I tried to protect that R. I kept telling her how much he loved her and this had nothing to do with her. I was overlooking HER natural feelings and she was angry and did not want to be around him! It backfired and she ended up more wounded. She also started to resent me.
Your D is 15, and he can initiate and arrange their time together. It is up to her right now to accept or reject the time they spend together. If she does not want to see him, it's ok. He is hurting her too. That does not mean it's over between them. You are anxious, understandable, but protect her feelings right now.
And just to share with you, my sitch and my teen D was burned by her bio dad too. Then H burned her. Very similar ages too. So I know first hand the fear of wanting to protect. Unfortunately you cannot protect their R--you can support it--but you CAN protect HER.
Cut him off. Go dark. If there is anymore family time, make other plans right now, and get out of there! You must do this to 1. give their natural R a chance outside of the M with you, 2. to protect your emotions from someone that is not safe for you, and 3. to start sending a loud message that you are moving on without him. You must value yourself and show your worth. We teach others how to treat us.
In terms of him having an affair and what his friends think. That is all mind reading. Mind reading never works! Actions speak louder than words and his actions indicate he has moved on to some other option and is deep in the fog. It's very hard to believe--I was shocked and in denial for a loonnnggg time---my H was the Nicest Guy! When H is in a fog, he is capable of more than can ever imagine. Protect your heart, protect your D, and assume the worst.
I know this hurts and I am sorry. It may get much worse before it gets better, but everything will work out eventually. Either he does a 180 and proves without a doubt he deserves another chance, or, you do not want thus man in your life.
Blu
“Forgiveness liberates the soul. It removes fear. That is why it is such a powerful weapon.” – Nelson Mandela
