Last time I did this was in my 20s and it was part of the bucket list that I'm glad I'm done with. I was living in NYC in the1970s and heard through the bar gossip that this guy had a monthly orgy at his apt. I walked up to him, got myself invited and went. Felt dirty right afterwards, as the years go by, memories make it better than it probably was.
I don't think you can talk your friend out of this. Once you get the 'itch' just let it happen, make sure he plays safe.
Craigslist is good, hanging out in chat rooms is another, or if he's brave, an adult bookstore with glory holes.