to whoever it was that said a 3rd of people admit to cheating, actually the figure is around 60%, so given that many won't admit to it reality is that the majority of people will cheat on a partner at some point, although obviously there are variables e.g. a kiss up to a full-blown long-term affair...
Judgement is IMO subjective, because while most people will consider that having an affair is wrong, and most wouldn't want to embrace the affairing couple and give them their blessing, many people do also recognise that life is rarely black and white, and that the view of "he obviously couldn't keep it in his pants," is a bit too much of a simplistic argument.
IMO what makes it worse in this instance is the fact that the dw has just had a baby. presumably there will have been talk in the office about the pregnancy, the man awaiting the birth of his child, he will have gone off on paternity leave when it was born etc? so another woman against the backdrop of the creation of a new family isn't one which sits well within most people's thinking.
But nobody knows what goes on behind closed doors, and not all babies are conceived into happy relationships.
And i somehow doubt that anyone would judge him less harshly for leaving while his wife was pregnant even if it wasn't to be with an ow, although being with an ow is of course not justified.
I certainly don't think the op has a duty to inform the wife who she doesn't even know. Different if the wife was the friend, but she doesn't have an obligation to her, the people in the wrong here are the husband and the ow.