In my opinion, You just never see M. again. Warn all your lifestyle friends that he's indiscreet. As far as calming your guy down, I have no idea. I'm too laid back to get upset over something like that so I can't tell you what will calm him down.
We have been with a couple who was too willing to discuss other playmates by name. As far as I know we haven't been burned by them, but once we figured out they had big mouths (and not in the good way) we stopped seeing them.
I hope it all works out for you.
Jeff