The mutual male friend, is not your friend.  He didn't need to say anything.  I question motive... Probably looking to move out of your friend zone into more fertile pastures.

This is not a smoking gun, just some blue banter between friends. Inappropriate, yes.  But context is important- it was said in private between friends, never meant for your ears, and he said nothing negative about you whatsoever.

Buddy boy with the big mouth is actively trying to fuck up your relationship.  Classic snake move. Sew discontent, be there to pick up the pieces.