What Should We Do...?

Brother in law kicked his wife out of their house so he could have a relationship with his side piece. The other day his girlfriend (the side-chick) walked in on him having sex with another girl. His current girlfriend broke up with him and told my husband. The woman my brother in law was caught having sex with has been a long running issue in the marriage with our brother in law and his wife. AND she has a fiancé... who has NO clue that this is happening. My husband knows this guy and feels fucking awful for him. Should he privately/anonymously disclose this to him? The way he sees it is he doesn’t want to ruin another marriage, or even worse, bring children into this situation.

Disclaimer: my husband has little to no relationship with his brother. He doesnt really give a f*** if it ruins his relationship with him further.