As the wife of someone who was having an affair with my "friend", I was so grateful when a stranger knocked on my door to tell me, out of the blue. I was the last to know and it seems the whole neighbourhood knew, including her young children, it seems. I've never once been angry at the man who knocked to tell me - he said that I was known as a kind person in the area, someone who always did good for others and he couldn't stand by any longer without someone telling me. He'd tried to persuade others who knew me to tell me, but they didn't want to know. I wish I'd known sooner. Needless to say, I've dropped many of those "friends" and neighbours. Cowardly to the end, in my book. Those who knew about my DH's affair but didn't say anything can't look at me. The shame belongs to DH, my now divorced "friend" and those who chose to look the other way.
So, BeenSuchaStupidCow, perhaps this wife is actually grateful to you too. If not, that's her choice. I know you're beating yourself up - my messenger was too but I reassured him he'd done the right thing. We keep in contact now and again via text. He even "advised" me on how to handle the whole mess, bless him! As for my DH and I, that's another story and, surprisingly, a happier one, which I won't go into here.
Take care of yourself, especially dating next time and I'm sure, if you think about it, there were probably clues he was married. There usually are.