Give your woman what she wants.
But, if you don't want to participate, don't. Sit beside the bed in a cuckolds chair, and watch.
Or, if your wife's lover-for-the-evening doesn't want a 3-some with you participating or watching, then sit outside the door of your wife's marital boudoir in a cuckolds chair, and listen (with the door closed).
Your wife wanting you present may reduce the number of potential candidates. Your wife should mention this while talking with her lover-in-waiting. If he doesn't want you participating in a sexual sense, you might talk with him and clarify that you can sit in a chair watching — or, if he doesn't want that either, clarify that you can sit in a chair outside the boudoir.