Been there - done that. A friend of mine hinted to a myself and 2 other friends on a night out that she as tempted by another man. We spent 4 hours that night practically pleading/begging her to not go ahead with the fling she was planning. We talked to her about the risks involved; losing her marriage, her daughters family life etc etc ....
She promised us she would knock it on the head and leave it as a fantasy.
Three months later she turned up my doorstep in floods of tears. She had embarked on the affair and had fallen in love. Again, I pleaded with her to end it, change her phone number, stop all contact, sort out her life with her husband and end her marriage if she wanted to, but stop the affair (for her own peace of mind, she was in a right state).
I had to keep all this a secret until the inevitable happened and she was caught. Cue separation, divorce, poverty, distressed child etc etc etc ...
The lover stuck around for about 3 months, and my friend says that the affair was a terrible mistake. She didn't listen to a word that anyone said to her, and I wish she had never told e any of it tbh. You have my sympathies!