My friend (30F) is cheating on her husband

I need advice with what to do!
My friend has been cheating on her husband for years with a lot of other men, they have 2 kids together and recently he has found out about one person she was seeing but is clueless that for years she has been doing the same.
I know she is still seeing multiple people at the same time and that their relationship is strained after he found out, but she has somehow made him believe that it's finished and it was a mistake.
He is so in love with her that he will forgive her, but even now she is still sleeping with other people whilst he is at work. She hasn't stopped and he is clueless.
Do I message him and have her hate me or do I tell her she needs to stop (which I know she won't because even now she is still doing it).
I won't what's best for all parties but especially their kids.
What do I do?
TL;DR my friend is cheating and I don't know if I should tell her husband


Source.

Sounds like you should stop being her friend