tdk, good job on not reacting. Like they say, fake it until you make it.
I’m not sure if I can you advice on what to say when he tells you about appending time with so called friend. Since you cannot prove that it is in fact the affair, all you can do is to keep your calm.
My H did something similar in our M. This was when we were still married and doing ok. He would spend time with his female friends once in a while. I knew they were just friends, but I was upset because I thought that he had no business spending time with women who I didn’t know or barely knew. I got upset and told him about it. Then it was his turn to be upset. He blamed me for being unreasonably jealous. This was also one of the breaking points he mentioned at BD. He blamed me for trying to prevent him from having good time with his friends.
If I would go back to these times in our M, I would tell him to have a great time with his friends, smile and go do something on my own. Your situation is different, because he already dropped the bomb on you and talking about D. In one of the relationship books I’ve read, the author mentions one of the possible responses to a spouse who is pursuing OP. The response is to tell the spouse that “I’m sure he/she is a decent person and a good catch”. You know that ow is not a decent person, because she is having an affair with a married man. And it all will come out eventually. Just stay away from it, so he cannot blame you for exposing him.
I really don’t know what else to say. I’m sure there are others here who would suggest you to enforce the boundaries. You just have to determine for yourself what these boundaries are. Meantime, stay calm and positive. Let then have their drama.
M:50 H:52 S28 (my S from previous marriage) M:17 + 3 BD: 06/12 S: 06/12 - H works in another state