Friend told me a secret that I cannot keep

My friend recently told me a secret that my morals cannot justify keeping a secret. She told me that she’s cheating on her husband and he doesn’t know about it. I have known her for more than a decade, and her husband is a great man who does not deserve this. I love my friend and am completely shocked by this behavior. I am completely lost as to how to proceed from this. Continue to keep it a secret or tell her husband?