i wouldn't directly tell the mutual friends--they obviously trust him for whatever reasons. you could try talking to him, that may stop him from talking to you, but it won't change how he is with your mutual friends.
i'd say let your friends figure it out. though i'm a bit of a bitch when it comes to people i don't like--so i'd prolly help my friends out in discovering this. if mr. shady told me my friend was involved in a threesome and the friend hadn't mentioned it to me--i'd make some sort of comment about it to my friend. when my friend was wondering just how the hell i found out about such a thing, i'd mention that mr. shady was talking about it
