My friend has told me she is/was having an affair, am I giving her the right advice?

Okay, so my best and longest friend who is also my cousin told me about 6 weeks ago that she had been having an affair. It’s a very long story but basically to begin with, I asked her not to confide in me with details if she was going to carry it on but i would be there for her no matter what her decision as long as she ended one relationship or the other. Her long term partner and father of her children is also a friend to me. So within a couple of days she ended the affair and told her partner that she was unhappy and wanted to end it. He begged her to stay and think about it. So after i suggested a visit to the doc to deal with anxiety and depression which seems to be in our family, she is feeling much better and happier and talking more to her partner. She has decided to stay with him and work hard to make it work. I’m very glad to hear it, but she recently said she was thinking of telling him about the affair because she felt so guilty and asked my opinion. I said not to, as I fear what he would do to the other man and also telling him would make her feel better and destroy him. I know I would want to know but I don’t think he would forgive her and is an affair that lasted a couple of weeks worth destroying a 10 year relationship? As we live in a small town, there’s also possibility of her children finding out? Am I wrong? Should she tell him?