Welcome home, Darlin'! I am so sad to hear about your daughter's situation. I had the same experience with my first husband and kept forgiving him and hanging on until there was nothing left of myself. It's normal to take the blame on yourself, thinking there is something wrong with you, when the reality is that they are just rotten husbands. As hard as it seems, the sooner she can get up the strength to kick him out and file for divorce, support and child support, the sooner will she find her own strength and be able to move on. Nothing easy in this, but if he's cheating now, he may have in the past without her finding out, and he certainly will do it again in the future. Philanderers don't stop philandering.
I'm sure this is so very hard on you, too, worrying what will happen to her and the grandkids, but it is by no means a new story, and many women have been through the same and come out stronger, healthier and able to thrive with her kids. That strong bond you share with your daughter is the most important thing she has right now to make some hard decisions. Keep us up to date and we will give you all the advice and support we can.
Kisses, Sweetie. Lynda
PS - I really, really hate him too! The dirty, rotten bas**rd! Let's help her to kick him to the curb. (My Ex went on to cheat on his next two wives, too. The last one solved the problem by becoming agoraphobic for a few years, keeping him too busy with the shopping, cleaners, dropping off and picking up the kids, teacher's meetings, and all the other "outside the house" things this stay at home mother used to do for the family. Good for her!)