If you were in my shoes

So, I was busy studying and browsing Facebook at the same time. I found out my closest friend cheated on her husband. It happened she deactivated her Facebook and scrolling through our mutual friends fb. She has another Facebook that I didn't know about and I saw her comments to our mutual friends. I found out her pictures, dates with this guy and they been dating for 3 years . She's been married to her husband for 13 years now.

If you were in my shoes, would you tell her husband, tell her first or just don't do anything about it?

Please respect my post. Thank you