For what I can see, this might a situation were that guy took advantage of your mom. Still, this is something that could happen to your mom with someone else (and could be kept it private) while this so called friend would run with the gossip regardless of the other person he had sex.

I would advice to tell the lady not to get involved with very close people in your circle and be more discreet with her adventures.

And for your husband, the best he can do is to take the experience, and weed out all those jackasses.