My mom cheated

So my mom cheated on my dad... someone wrote my dad a letter back in February saying that the same guy that my mom is with had an affair with his wife and told him who the man was. So only recently did I find out this was going on and I feel bad that I didn’t catch on sooner. I had always has suspicions of it but I couldn’t figure out why but looks like my gut was right. Through out all this my dad has been nothing but kind to my mother and is trying to win her back even though she doesn’t deserve his love (in my opinion) but she’s not having it. It hurts me when I go up to my dad and I can tell that he’s hurting and I give him a hug and he just breaks down on my shoulder. I love my dad to death and I wish he wasn’t going through this. I hate seeing him like this. How can I help him? I try to be there for him as much as I can but it breaks my heart to see the person I look up to the most break down because the person they married 20 years ago found someone else but won’t get a divorce or try to make it work. I need some advice and prayers for my dad and our family please.