I have a lot of experience with couples threesomes going back 29 years (shit, that makes me sound so old!). There have been great experiences and some pretty bad ones. Some pointers:
1) Make sure your wife and you have the same expectations from the guy and the encounter. Couples tend to stick with familiar roles, and seeing your wife in a different sexual light (more aggressive, for example) might trigger an unexpected anxiety in you.
Example: Last year I wound up in a spontaneous four-way involving a casual fuck-bud and his best friends, who were a couple. In the moment it was totally hot, but it turned out that the dynamics of the couple was undermined when I topped the habitual top and the bottom freaked out.
2) You and your wife should set the limits with each other first, then explain them to your friend.
Example: I had a partner in the 80s with whom threesomes were very common, but he had a thing about the hook-up coming in my mouth and made a terrific scene the first (and last) time I did it. I didn't know until he'd told me that he considered that specific act to be off-limits.
3) Make sure all participants are attracted to each other. It's one of my firmest rules, but I'm unsure how it would work in a totally hetero MMF three-way.
Example: My very first threesome was with a couple in their late 30s who had been together for many years. I was 18. I felt much more attracted to one than the other, and the one I felt less attracted to was excluded from much of the fun. He wound up fetching drinks and an ashtray before he basically walked out.
4) Make sure everyone's on the same energy level.
Example: I used to play frequently with a couple who were really comfortable with me and the amount of rough (but playful) enthusiasm I bring to sexual encounters. It was great because we were three people in total synch. Based on the strength of our rapport, they suggested that I hook up with a couple they played with. The chemistry (attraction) was there but one of them just wasn't on his game and the session was over in less than 40 minutes (which is just a warm-up for me).
5) Discuss your feelings honestly with your wife afterwards. Make sure you both enjoyed it, or discuss any misgivings either of you had. Was it an experience you'd like to repeat, either with the same guy or someone different? Why or why not?
There's no reason why this shouldn't be a fun and enriching experience for you both. Just keep communication as open as you can and really explore the fantasy (and reality) of what you're doing.