Similar here to DS and his dad.
He didn't cheat, we separated through DV that DS witnessed at some point. He has also been unreliable to him, although it improved massively after the first year, and emotionally tiring (not to say abusive) when contact happens. He is a source of stress for DS, not what a parent should be.
The only thing she wants from him is a full understanding and apology for his behaviour.
This resonates with me. DS has a big problem with his dad in that he keeps denying and minimising his actions.
He also refuses to engage DS directly by email or text, or even ringing him, other than the skype calls they used to have, despite my strong suggestions. I refuse to force DS and can't be bothered to facilitate (read: strongly encourage and mediate) any more.
Of course I'm not emotionally invested in keeping a relationship with exH, so, it's not a big issue for me, other than the sadness of seeing a father-son relationship degrade to this point.
Where we live, at 12, which is soon, DS will have the right to appoint his own solicitor in parental disputes, and he is quite pleased with that.
Keep talking to your DD, listen to her reasons, be frank about your own feelings too. But, ultimately, their relationship is for them to sort. As is yours with him too, although, as others have pointed out, I'd use his reaction and actions towards your DD as a guide in relation to what kind of person he is, and what kind of husband he can be.
How was your relationship before the affair came to light?