When I was in your position I found it so, so hard so I really sympathise with you.
I'm naturally a "fixer" so found it very hard to bite my tongue, but I managed it and am glad I did as I know if I went on an anti-him rampage which I want to because he is a smug cunt he would eventually isolate her from me.
I had an initial big sit down with her fairly soon after she found out and said clearly what my position was - he is abusive and cruel and she deserves much better and I personally think that she should leave because I love her and want her to be happy.
But I then said you know this is how I feel so I'm not going to keep saying that every time we speak about it, I promise to listen and give you a cuddle and answer questions honestly but not make you feel bad for staying.
I did reiterate that if she did leave at any point, she has a place with me for as long as she needs and can call any time any day no matter what.
It's agony but I know what she's going through is a zillion times worse and I can't risk pushing her further away by berating her for staying.
People like him twist post affair relationships into "us against the world" so that anyone who doesn't support the relationship continuing is painted as someone who doesn't support the wronged party. It's awful and so manipulative.
Poor you OP and even more so poor her 