There is no definite answer. I didn't tell because I thought that silence was the best thing for the AP's wife and kids. Then someone told her anonymously anyway and I now believe it was better for her to find out. Possibly also better that it wasn't from me, not sure. At least I don't have to blame myself either way.
When I didn't tell it was on the basis that the AP would focus 100% on his wife and family. (He didn't, but that was my condition... glad I didn't have to pull the trigger myself though.) I don't think you can be sure the sex is over, but even if it is, it is still an affair, and his wife is being abused. She is doing stuff every day and every night that she would not be doing if she knew the truth. That bites hard later. I guess I am saying I have come around to the "tell her" position, over time, and especially if her husband is not devoted to fixing his mess.
Maybe it is okay, maybe it will be okay.
BS, d-day Feb 2017, 16 mth affair.