Should I tell my mother her husband is cheating?

So I live with my mother and step father. They've been married almost 20 years. My step father has been in my life pretty much as long as I have been alive. I'm 22. Anyways a lady on Facebook contacted me and all but came out and said she and my father were having an affair. She had no profile picture, and posted few things The posts she did have were promotions of my step fathers business. Quite frankly i think is a fake profile for a real person but idk. Also Thought it was a scam until she began to tell me things that only he could have told her. I don't want to get in between their marriage, however I slightly feel like this needs to be addressed. Should I confront my step father, tell my mother, or stay quiet? **Any other advice you have will also be helpful.