People nowadays have no compassion for others or desire for something to be "just"
What is considered "just" changes from culture to culture and with varying moral perspectives in our own culture. Not everyone considers it compassionate to tell someone their spouse is cheating on them. Some people consider it interfering.
Cheating on your spouse is morally wrong and if you don't want to get found out, don't do it!
Some people have open marriages and agree that they can both sleep with other people. Since they don't view this as morally wrong, and they both agreed to it, then it's not cheating. How does this woman know what arrangement the husband and wife might have privately? She admits she doesn't know them that well. Perhaps they are swingers? Perhaps the wife has a boyfriend too and the husband is OK with it? My point is that it's a gross assumption to believe that all married people have the same sexual views in marriage as we do and have to follow our "rules" about it.
It is important that people care enough about the victims of this selfishness enough to act.
Again, "victim" is a characterization of the wife that she may not share about herself. There are many kinds of selfishness in marriage many have nothing to do with sex.
Its all ok saying its none of our business...well it is our business to help someone out who is being made a fool of and who is being put in physical danger (std's etc). Also people wasting their loyalty and their life on someone who is a cheating b*stard.
One cannot be made a fool of by another person. One can only make a fool of oneself. "Physical danger" is another gross assumption. We do not know the disease state of the persons involved (if any) or what protective measures they use. There is no reason we should know. It's none of our business unless we are planning to have sex with them.
I've been at both ends of the spectrum....when I was single (and newly divorced and screwed up) I had a very brief affair with a married man and I deeply regret it BUT I WISH SOMEONE HAD TOLD THE WIFE because this guy was promising me he was leaving her and never did. At least if the wife had found out the whole thing would have come to a head and been sorted out and I would not have been led a merry dance by the guy.
I can't help noticing that you don't say you wish someone had told the wife out of unselfish concern for her victimhood. You wish someone told the wife so that you would not have been led a merry dance. Ironic!
I've also been in the position where my husband left for another woman and I hated the idea that people knew and I'd been made a fool of.
I suggest that your husband made a fool of himself by his dishonesty, not of you. Being worried what other people think of you is about "ego image" and has nothing to do with "justice" or "morals". Many people confuse the two in their attempts to rationalize their behaviour.