If you see your friend's husband out with another woman, do you tell her what you saw, or do you not say anything about it (a lie of omission)?
After all, you may not know the circumstances. It might be completely innocent, and she might even know about it. Saying something might make her think you are trying to destroy her faith in her husband. That would affect your friendship.
But if you say nothing and she later learns you knew and didn't say anything, that will affect your friendship, too.