When your friend cheats

To cut the story short: my husband's really good girl friend who is getting married in a month is cheating with his family friend.

We just found out. She is in a relationship for 10 years. She's not a kid - 37 years. The family friend is around 55 - has 3 kids and even grandkids....

Apparently it's being going on for over a year, and he now moved just a few streets away from her. She and her fiancé aren't in the same country right now - he got a new job elsewhere and the plan was for her to move after the wedding.

It's not just that either, the other guy's ex gf found out and had fights with our friend telling her to leave him alone- that's how we found out too.

Now, we also know her fiancé, my husband is friends with both of them for 15 years and we traveled with them both, our families (parents, siblings etc) all know each other and are all friends, we visit them and are in contact a lot.... very messy situation!

My husband now decided he won't go to the wedding and wants to let her know why. I obviously don't approve of what she's doing but also don't think we should interfere in any way. It's not our place. And also, I would hate to do something that will hurt her parents they are so nice and don't deserve to feel bad bc of her bad decisions.

But don't know what to do here?? I can't just continue like nothing happened- a few people know as well and everyone is thinking of not going to the wedding and kinda not having a relationship with her anymore.