This can be a good learning experience for you. For months and months you bashed your W up and down for feeling the "high" of a new person. And now that you have experienced it yourself maybe you can better see where your W was at and where she continues to be.
It is *very* easy to get swept up in the "magic" of it all. Somebody to pay attention to you, somebody to tell you that you are smart and charming and funny and attractive. Somebody to talk to in a "non DB'ing" fashion. Now can you see why your W was so drawn in to other men? Now that you have experienced this yourself have you gained any perspective on the situation as a whole?
I do agree w/Trent. Either be done with your M and file or not be done and keep trying to DB. But don't involve a third party because (A) it will only hurt one more person and (B) your W will feel things are "even" now between you and her since you BOTH have engaged outside the bounds of marriage.
Tell us what you learned from your experience last night. Have you gained any new perspective on where your W was coming from? Have you gained any new perspective on yourself and your situation?