No doubt, I do not know too much about OM situations, but do you know this guy, does he know you know? If I were married and having an affair with another mans wife, I would have a hard time looking that guy in the eye, Come to think of it, I would have a hard time looking in the mirror.
Does he know the ramifications of his actions, two broken families, loss of two careers?
Do not push these wedding plans in her face, but if the subject comes up again, why don't you say, I will email you some information about some ideas I have been looking into.
I think she is well aware of what is expected of her if she chooses you and your daughter.