I was the wife in this scenario.
To those saying she probably knows, I'd hazard a guess she probably doesn't. I didn't. Most people don't until something happens like OW gets in touch, or falls pregnant, or someone else tells the wife. It is so easy for men (or women) to cheat when they regularly work late, or away.
Letting her know isn't acting as morality police. It is allowing her to make decisions about her and her dcs future herself, with all the facts in front of her.
My own particular public humiliation as a result of no one having the decency to tell me, was attending an overseas with all his colleagues when they all knew. Looking back it makes me want to curl up and die a bit. I was so happy (I thought). I couldn't understand why one particular colleague went weird when I was around and couldn't look me in the eye. My DH said he was an odd ball. I took that at face value. I thought people a little unfriendly, but figured it was because they didn't know me. No, it was because they knew HER. Not one of the cowardly litte shits bothered to tell me anything, and I hate them for it.
I guess you could say it was not their business. Or I 'probably knew' because yeah, most women just say 'hey ok, that's fine honey you carry on' when their DH is shagging around 
. Or it wasn't their job to act as the morality police. Or any other classic opt out clause that comes up on here.
The reality is pretty much everyone who has had their marriage torn apart by an affair will say they wish they'd known earlier. Women have gone on to buy houses, have dcs, give up their job to follow a DH's career location move etc when they absolutely wouldn't have compromised their independence had they known.
So yes, I'm venting, but FGS tell the poor woman so she can get a SHL and stop wasting her life with a piece of shit DH.