Not sure if you are trying to get absolution from the forum by raising it here.
You and your wife seem very comfortable with the arrangement whereby your wife is able to engage in sexual activity with other men with your blessing and she seems happy to do so.
If you didn’t know this person was married then you would not be having these conflicted feelings you are, but there does appear to some extent a laissez faire view when it comes to you and your wife with regard to married men.
So I guess that knowing he is married and your wife wanting to continue to see him for sex, are you able and willing to deal with the fallout as others have said if the wife finds out, because if she does then I very much doubt that you will be able to contain the fallout and your lives might be laid bare for others to judge. Would you be able to handle that?
If you would then you can only be judged by your own morale compass, if you are fine with it, then who is society to judge, we may have differing opinions as to whether we would deem it acceptable but it is really none of our business.