What you have to realize, is he is now facing the issues with the OW that he has not been facing. She is threatening his world by saying she is going to move behind you guys.
Think about this Cathy, how would that make you feel? What if you had been the one to get involved with someone else and you wanted your family back? They were threatening your world by trying to get into it. You have a son that you don't want to hurt and this woman is threatening it.
Now, he feels that his only answer is to give you guys up in order to protect you guys. He does not want to give you guys up, but he will sacrifice what he feels he needs to in order to protect it.
I know that sounds funny, but that is why it is so important for you to continue to validate and be there for him. She will eventually turn him against her because of this. Remember how it turned him off to you when you were acting this way, well she is going a step further and that is not cool.
Just keep being his friend and listening to him. He is full of shame and it is starting to show. He has to hit rock bottom and he is getting closer and closer. The more she pushes the further he goes. The more he can come to you, the better off he is because you are showing him the way. The Lord is going to use you and you have to open for him to use you.
