Puppy is right - follow your gut and don't be afraid to expose this. I took it a step further - I not only caught my W in her EA after her claiming just 'friends', I also told her mother and sister and her closest friends,whom she had also been lying to about it. I kind of went too far there, but in the end the cockroaches scurrying reference is true. If I had only kept it bewteen her and I she probably wouldn't have done much except lie some more about it going forward. I imagine in most cases exposure and keeping it between spouses is the right course of action.
In the end she drifted back to the EA, but if I hadn't exposed it when I did and at least slowed it down it would have evolved to a PA very quickly.
I actually kind of blew up and our kids overheard one of the discussions, so she even had to admit to S17 and D15 that she had a 'friendship' that went too far and dad was really mad, etc etc. Her IC told her to explain it this way.
While I am sad that my kids were exposed to the sordid mess, at least it made her consider her behavior in the light of the real world in front of the people she loves (like my in-laws and some of our closest friends).
ME/XW:47 S21, D19, S15, S14 M:21 T:26 W moved 6/10 I filed 7/10 D final 4/12 remarried 8/12 W wants to R 12/10 and 4/11 but I decline