One piece of advice I received on the board was not to react/act immediately. Wait 48 hours and see if you still feel the same way. I've gotta admit, it kept me from doing a few stupid things.
IMO - I don't know if contacting OW is a good idea. I understand your point - what if she doesn't know he's married and ends it. But you need to think about what other outcomes might happen. Maybe she doesn't know but when she approaches your H with the info, he gives her a nice story...like you've been separated for months or it's been over but your still married because you need his insurance...etc... The number of stories he could feed her are endless. Just think long and hard before you do anything.
Also, don't give her too much credit. Not everyone has the same morals and beliefs...although personally I would never date a married man, not everyone feels that way. The OW in my sitch is married. My S wrote her an e-mail calling her a bi!ch and telling her she is destroying our family. You'd think most people would stop the affair knowing what's it doing to a child. Nope...not her. Ticks me off every time I think about it.
I know this is living he11 and I am so sorry you are experiencing it. You are definitely not alone, as you can see from the amount of people on this board. Just hang in there. Have you seen a C yet?
One more thing...I don't know what your religious beliefs are but I found my way back to Church and to God after this began. It made all the difference in the world. It helped me control the anger and vengeance I felt (especially toward OW).
Me: 39 H: 39 S: 15 M: 18 years Bomb: 6/3/09 H moved out: 10/15/09 H moved back:5/30/10