This is very similar to my sitch as well. In addition to not only having a A, my MLC-WAW was the OW in the A. Clearly, as you suggest, she was a piece of a$$ to this guy. I did some snooping and confronted her with this info this past Sept. to which she stated it was easy to get involved with someone where there would be no commitment. How sick is that? The ironic thing is now there is commitment as the OM has evidently dumped his girlfriend.
Me: 48 Ex-W: 45 M: Nov '96, together since Oct 93 Bomb: on 10-yr anni - Nov '06 OM Separated: mid-Feb '07 Divorced mid-July '08 One daughter - 28 XW living w/OM