Staying with cheaters is not popular on Mumsnet but honestly, you can't possibly judge until you've walked in her shoes, and no one knows how it feels until it happens to them.
I knew deep in my soul I could never forgive someone for cheating, but then it happened to me and I stayed.
I stayed because I believed he genuinely and deeply loved me and that what happened was the result of the mess he was inside. I recall so clearly when I found out, that it seemed like my entire universe had collapsed on me but I instinctively knew the person standing in front of me genuinely loved me and didn't love the other woman.
When we say "I'd never stay" I think we work from the basis of a healthy person. A person like me, for example, would only cheat on my husband or partner if I truly didn't love or want them anymore or value them in any way. Even then, I'd just leave rather than put someone through that kind of pain and humiliation.
When you really learn more though, cheating on someone you love (or anyone really), shows a staggering lack of respect and value for yourself rather than who you're with. It's a degrading act. People who love themselves don't do things like that.
So it was in that moment, we both realised, my DP didn't love himself. The affair was a catalyst for big changes. He got therapy and unravelled his "whys". I found out things about him I'd never known and we became closer than ever. He devoured books on the subject. He went through a pretty public nervous breakdown. He committed to me in ways he never had before. And he became a better person for having cheated on me. He grew from it and became someone he respected and loved.
I think until the moment I stayed, he didn't know what it meant to be loved, because going through that was the hardest thing I have ever done.
It was literally years of work. Mostly on him. Recovery for me was a long road. He would tell you now he is glad something big happened that made him make those changes, and he is grateful I didn't give up on him. But he would wish it didn't take that for him to get there. He knows what happened wounded me in ways he never anticipated, and it was a terrible process of grief and change for me to heal.
Your friend won't know how hurt she is or how damaged she is yet. It takes a long time to recover. Whether she stays or goes, that damage is now hers. She has no choice about that. Leaving doesn't resolve that pain.
The easiest thing in the world for me would have been to have walked away and met someone else nice and put it down to my DP being a terrible person. I did the harder thing, but in the end the right thing for me.
Reconciliation is a lifelong process possibly. When someone does this to you, it changes everything in your world. You lose some things, you gain others. It killed me in a lot of ways and there are things which still hurt so much. I am not sure life always plays out in fairytales though.
I know I have a partner who'd lay down his life for me and who makes me happy. I will carry the pain of what happened forever though. It's complicated to explain, but healing doesn't mean the pain really goes away.
I am sorry for your friend. As previous posters have said, the shame she will feel will be magnificent. Stick by her whatever she chooses to do.