what would you do?

I know that most of my readers are wives and/or mothers, so I’d like to know your opinion on this. I know this woman who I’m not that close to but a very good person. Then, one time I saw her husband with another woman. They were dining with his arms over the other woman’s shoulder.

Should I tell her?
It might save her marriage.

Or should I just ignore it?
It might save her marriage.