You may want to do a confront or a re-confront with her parents, using your evidence if you have to. You may not be able to stop the affair at this point, but you can probably get her to stop the DECEIT, which -- to my view -- is important. I had to do the same thing with my wife, as she was lying to her parents and our two adult daughters about her affair. I gave her a couple of months to come clean about it, and then finally threatened to show them my evidence if she didn't start at least telling them the TRUTH (after all, if their "love" is so "special," why not shout it from the rooftops???).
The thing is, if you are to move forward as a divorced couple, co-parenting your children, you CANNOT built that family upon a foundation of DECEIT. I told my wife "We've always told our children not to lie; how can we hold ourselves to any less of a standard?"
Are you SURE her parents are condoning a full PA, or are they buying into the "just friends" thing? My in-laws waffled back and forth several times, and seemed to believe whatever argument they heard LAST. It's understandable that they'd want to believe the best in their daughter, but sometimes confrontation and exposure has to be repeated with each incident.
