No.
Should I tell my friends that another one of our friends thinks they are fake?
No. There's never a good reason to hurt anyone's feelings.
Just stay away from the one who says those things. They will stab you in the back quick if they're already talking about their own friends that way.
Your friend will find out soon enough.
Are you a pot stirrer? If not .. Then don't tell them.
Absolutely not! How would you like that if you told someone something in confidence about another person and then they turn around and tell that person? If they wanted that person to know they would have told them. Plus, if you do tell there is a good chance you can lose both friends.