I'm going to ask a question before giving an opinion...and I am a little surprised no one else asked. Are you, and if so why, secure believing they haven't fucked since you two became a couple? You called him one of your oldest friends, haven't they been in each other's orbit and there is a hint she's at least thought about outside of your play/conversations.
It's a delicate question to ask of either of them. BUT if he knows you're aware they were fuck buddies, I think you should set it up with HIM and surprise her with it, and not ask that question at all. If they did, they'll tell you eventually once they see you can handle it.
