I'm a little unnerved that you were discussing sex as a threesome. I would never discuss my marital intimacy with a friend of my husband's or a girlfriend with my husband a partner to the conversation (or any other time).

I think that your wife's girlfriend was part of the conversation could have further amped up your imagination? Did you even discuss (with your wife alone!) how your wife felt about the experience and why she never pursued it again? I think what you believe the man experienced is all in your own imagination/head and not necessarily what actually happened.