You can blame the man, the other woman and the acquintance who knew and said nothing. It's not a matter of either/or.
Frankly, I wouldn't trust you either because you knew and said nothing. In my view your relation with these women is more than strangers or random people. You are still in their group despite no real need now that children are grown up. This indicates some sort of relation beyond a basic acquintance for purely practical reasons. You participated in her humiliation by keeping your friend's secret whether you intended it or not.
The comment about not trusting you around her husband is really stupid.
Loyalty to friends is not a carte blanche. Would you not tip off the police if you knew your friend caused criminal damage? What about if they killed someone? Does it depend on what they have done and to whom? Of course, some would turn a blind eye.. morality is on a sliding scale it seems.
I find it mesmerising how many a times replies on MN were largely in favour of being told when asked whether they would want to know if their husband is cheating yet here the replies seem to be backing up your choice of silence.
It seems the angle from which you project your question, for what is fundamentally the same problem, yields different answers.
People have their own morals and views upon which they make their choices... i would respect their view as i'm sure you would like yours respected and find a new group of friends who share your values. I would leave the group out of empathy for the poor woman if nothing else.
And before i get jumped on about the husband and ow, well they aren't on this thread to get a piece of my mind, so please don't make it a man/woman thing - it's really not.