Voice of experience here. My wife and I's first experience with someone outside of our marriage was with my best friend. My wife had mentioned that she thought he was "hot" and wouldn't mind sleeping with him, but that she would never cheat on me, etc.
I explained to her that I was ok with it if he was, and I would talk to him about it sometime in the ner future. Well, after she and I had many discussions about it, I finally got the balls to set it up and ask him.
We had him come over for drinks one night. We were all sitting around playing cards and board games. Then, it was getting late, (about 1am) and we were all hungry. My wife was drinking more than us, and so he and I went to Taco Bell for something to eat. While in the drive thru, I looked at him and said "So, I don't know if you noticed or not, but L (my wife) finds you pretty hot. He laughed and said "Oh? I hadn't noticed, but she is pretty drunk tonight. haha". I looked at him and asked "Well, would you like to sleep with her?" He about fell off the earth! LOL His mouth dropped open and he stammered a bit and said "HUH?!?" I repeated my question to him, and added that I was ok with it if he wanted to. He said he wouldn't mind, but didn't want to piss me off. I told him that I would make the first move back at the house. Well, he couldn't have driven that little truck any faster to get back to our house. In fact, he almost forgot the food! LOL
We got back to the house, and to ease his mind, I helped my wife get a little "more comfortable" by helping her to remove some of her constrictive clothing.
This relationship lasted about 2 years until he got married. At that point, he cut it all off citing his wife as the reason why. He said she didn't know anything about his sexual relationship with my wife. So, out of respect, we never slept with him again. We never pressured him about it, and generally never spoke of it. He said his wife wasn't into that type of relationship.
So, my advice?
Invite him over for some booze. Pop the question early enough that no-one is drunk. This way, you can CLAIM you were "pretty messed up last night" if you HAVE to, because he said no, the next day. But it also helps to loosen up a person a little more.