Wow, I could have written your exact post. It took me weeks and weeks before I finally told the OM's spouse.
I grieved over it, felt so much guilt over it, discussed it with my WW. My WW was also opposed to it. I thought she was protecting the OM. Maybe in a way she was, but, after I did it, a great weight was lifted off of me and my WW and I are doing better, making more progress than before. At first, my WW was super pissed, threatened to leave me. I said hit it, if that's all it takes then I'm done. After a while, she completely understood.
It's not revenge, it's her owning her shit, she had a hand in your pain, not just your BF. Her BS DESERVES to know. Just like if he had found out and not told you. How would you feel? It is the RIGHT thing to do, no doubt about it. It may save him some pain later on if he decides to leave her if she is such a disgusting serial cheater. My wife's AP is also a serial cheater. The other BS appreciated me telling her.
Hope this helps!