I agree it's best to not give advice. Also maybe save your money to help your daughter if she finds herself on her own down the road.
My ex cheated on me and my Mom tried to give advice but I resented it. I was still in denial and wanted to see the best in him. It often backfires too. It's best for adult children to talk to their friends or a counsellor not their Mom. It's also best for Mom's to not confide in their adult children about their marriage too.

As for the female scoundrel, why do people always blame the other woman. I didn't. The guy as you said tells this woman terrible things about your daughter. My ex told his affairs that I was crazy and on medication. It's the husbands that you should be angry with, they deserve the blame. The other woman should no better yes, but she is not the one breaking the marriage vows. Guys should keep it in their pants.

If he is a serial cheater it is best for her to be on her own. She may not think so now but it will be.

For now my only suggestion would be to encourage her to seek counselling and talk to her close friends especially someone who has been there. Your being there for moral support is all she needs from you.

Kate