I had a girlfriend that found out she was the OW. At first she broke it off, but like a moth to a flame...
My friend said many of the same things your daughter has said. What a great guy he is. He and his wife are never intimate. Etcetera, etcetera.
I had a very serious talk with my friend. Mind you, this was before I discovered my WH's crap, but I think I would still say something similar.
I told her that even if he is Prince Charming, even if he's honest, caring, and your best friend, even if he leaves his wife to marry you and stays honest, caring, and your best friend until the day he dies, the two of you will never have a Happily Ever After.
My friend's family is very traditional. She, herself, was D because of her WH's A. I reminded her of how she felt about that, how it affected her family, her son. I asked her how her family would react to the situation, how her son would react. And I made her say it, outloud, knowing that the answer was that they'd never, ever accept him. No Xmases with family. No Bbqs. No Saturdays at the park.
I then asked her how his kids would react to her, the OW that ended their family. I made her think about and be honest, and most importantly, say it outloud so she could hear the truth from herself.
Because the truth of the matter is: Nothing that begins with lies and deception, nothing that causes family destruction and pain, nothing that begins in darkness can possibly end in blessings.
A tree only produces one kind of fruit, and that fruit depends on the seed planted. Remind her that what she sows, she will reap. Make sure the seeds you plant today don't produce the hemlock you'll die from tomorrow.
Anyway, that was my big talk with my friend. And she did stay away from him for a while. Then I became a BS, and we're no longer friends.