Had this exact same situation with a close friend of ours many years back. His marriage was cold and due to end soon, and I knew he always had the hots for my wife. The three of us were all close friends from college. We went on a ski trip together (without his wife), and while we were there I explained his situation to my wife. I just came straight out with it, saying I thought she should give him a blowjob to help him out. She said "really, would you be OK with that?" and of course I said I would. She just laughed kinda embarrassed but I noticed she didn't say no so I knew she would think about it. Later that day I was riding the lift w him and he was b*tching about not having had sex in forever, and I said "you should let (my wife) give you a blowjob!". (I find the direct approach is best
He did the same, laughed kinda embarrassed w a shocked look on his face but didn't say no.
On our last night of the trip we went out to a bar apres ski and went dancing. She wore a button down top w no bra and the more we drank the more she unbuttoned her top. Her nipples were clearly visible as her shirt gapped open when she danced and she even flashed the band at one point (she can be quite the fun little exhibitionist when she's had a few). I should mention she's very attractive and has very hard dark nipples, so she was getting quite a lot of attention. My buddy (and everyone else on the dance floor) couldn't keep his eyes off her tits and she was clearly teasing him.
At the end of the night we stumbled back to the condo and we went up to our room while our friend was downstairs. She was swaying a bit and after she took off her top she couldn't get her necklace off and asked me to help. I saw my opportunity. I pretended like I couldn't get it unhooked so I yelled down to our friend to come up and help. He walked upstairs and saw my wife standing there topless in just her jeans. He was speechless but he quickly adjusted, and clearly knew where I was leading this. It was a huge thrill to me, he had never seen her fully exposed in all our yeas of friendship and here she was brazenly exposing her tits for him! Incredible turn on.
He helped her with the necklace and then I said "lets get her into bed". She sat down on the bed and laid back, and my friend unbuttoned her jeans and we peeled them off together, so she was totally naked with her shaved pussy exposed for him. At this point he and I both just stood back having no idea what I had started or where she would take it. She didn't hesitate, she sat straight back up and practically lunged for the buttons of his pants. She whipped out his cock and immediately went to town sucking him. I could NOT believe it!
We all got naked and I was so hard I couldn't wait. I fucked her from behind while she was bobbing up and down and deep throating his cock. I was so turned on I blew my load in her pretty quick. But he had whisky dick and she sucked him for what seemed like a very long time. Finally he realized it was pointless to let her keep sucking away, so he turned her around and started fucking her doggy style. I remember thinking "wait a sec that wasn't part of the deal!" but it was too late, they were totally into it and I couldn't stop it. I was actually loving it. After fucking her for awhile he pulled out and she turned around and took him right back in her mouth. He came down her throat and she swallowed every drop (she always swallows).
That was our first time in our marriage she'd ever gotten any strange cock, and she's fucked him at least a couple dozen times since that day. Its always been awesome and fun.
So my advice, for what its worth - DO IT! But only if you're all really good friends and you talk about it first. Things were briefly a little awkward at first the next morning, but we all agreed that the sex was just fun and our friendship mattered most. That set the ground rules, and if it happens it happens, if not no biggie. Thats what kept it cool ever since.
Secondly, although she says she would only lap dance, masturbate, etc. be prepared for the possibility they will end up fucking. Its the natural progression. If you are OK with that, fine, but if not be careful. Either way, talk about it and set the ground rules and boundaries before he gets there or you're asking for trouble and drama. And nobody needs drama!
Have fun!!