Couplers is correct, do not just jump in though.
Discussion on all boundaries are a must, once you and the wife are 100% on same page have her approach a friend that you both agree on or you approach a friend you both agree and discuss with them the fantasy and if the person is interested The Husband/BF should be the one to get the feel and explain everything to the friend and only when he is 100% confident that the situation is doable, then bring the wife in to for a discussion dinner/meet type thing and set a go date.
Make sure she knows that she can withdraw at any time for any reason no questions asked. (however make sure you are sure this is what you want to do, sometimes it ruins relationships when fantasies become reality so trust is a major issue on all your parts). If done correctly it can be an amazing experience.