9 times out of 10 it's guys with their own agenda and a girl who liked the attention. Are they friends of your relationship?
If they don't treat you and your bf like a unit and if you don't hold them to it... it's not real friendship.
Every couple should read this book.
YTA, Leave it alone. If it had happened a LOT, or over the course of a long time, or was very recent, or they were having a lot of current marital problems, then maybe you should tell. Or more specifically you would tell him he needs to confess or you will tell. Always give the cheater a chance to confess.
At this point, years later, a one time mistake like this should just be left alone. I would suggest to him that he read this book: NOT Just Friends What he got himself into is a common trap these days, and it could happen again if he doesn’t take precautions.
One more thing - Esther Perel has an excellent book and podcast on her marriage counseling - including helping couples deal with exactly this. She is wise and worth listening too:
https://www.estherperel.com/podcast
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Lots of good books - you might want to try:
by Shirley Glass and Jean Coppock Staeheli
Please read this book: https://www.amazon.com/NOT-Just-Friends-Rebuilding-Recovering-ebook/dp/B00120955S/ref=nodl_?dplnkId=d3fccade-21f1-4e76-9d84-de265357a5a4
It’s exactly what you’re going through.