Would you tell your friend if you saw her husband with another woman?

I would want to know. But tell her gently, privately, and only what you actually saw, not what you assume.

Be prepared that she may turn on you, not him. It happens. Only you know if your friendship can hold it.

I stayed silent once and I still regret it. Whatever you choose, choose it from love, not gossip.

It actually depends on the level of friendship, and even if I am going to, I will do it anonymously