We have been in this situation. I told my buddy that my wife and I played out a role play involving him and that it was hot. I told him that the MFM fantasy figured into our night-time fun frequently. This put us out on a limb a bit, but not too much. He could have declined to take the bait and it would have been less awkward for all than putting him in a position where he was forced to reject us.
Thankfully for all of us, he was indeed interested, and we had some wonderful threesomes.
If that approach is too forward for you, maybe you could strike up a buddy to buddy conversation about how wild women can be with their role play ideas. Tell the story about how she always digs MFM fantasies. Tell him that you would seriously consider doing it if you could find someone you trust, etc.
My idea was simply to bait him and allow him to show interest. If he showed no interest, we could all walk away like nothing happened.