Puppy, Yes, she was having a PA. It started with a guy from her work that started flirting with her, then texting, to phone calls, then PA. Her H found out about her having feelings for another man right before the PA started. She said she needed to get her head straight, and her H supported her on this. Well that was the night it turned into PA. She never denied the A, in fact told her H that night. He begged her to stay, and she did for a couple of weeks, with her flaunting the A in his face. Of course he was heartbroken, and he say's it's the hardest thing he ever had to do. After a week or two my sister decided to move out to continue the A. This is where my brother-in-law started pursuing her, and in there case it worked. She finally decided that she loved her H more then this other man, and dropped him and went back to her H. This lasted all about 6-7 months. They are still struggling with trust issues, but are working on it and they both say they are happier then they were before.
So maybe a strong stance -- that day -- may have averted the PA, MONTHS more of continued emotional and physical involvement, etc., etc., etc.?
And what kind of emotional toll did "flaunting it in his face" take on your brother-in-law? We may never even know, but as a man, it makes me shudder.
Look, you can DO whatever you want -- it's your sitch. I'm just saying that in studying THOUSANDS of affairs the past three years, I've never seen this tack work. Never. Even Hope4Us's notoriously-long, patient, "standing" for his marriage was accompanied by VERY strong confrontation and exposure on the front end (and re-exposure midway thru), including to his own children.