She knows that I do not like him and suspect her of having an A with him. Why on earth would she insist on bringing him up?
I would say she brings it up to try to justify to herself that it is ok that she is doing it. You know and she knows that there is no justifying it. But she tries will continue to try to do so. That is the only way she can deal with the guilt. By you not falling into her trap of trying to get your approval, in a way, by you talking about it with her, will only continue to fuel her self doubt and guilt. If she can get you do fall into the trap of discussing OM, then she feels more justified in her decision to have an affair and the guilt begins to fade.
“Do you want to be RIGHT or want to be LOVED” “You have to have a life to share a life with someone” “When you stop resisting, you start learning”