Should I Reveal My Friend’s Secret?

I've been close with a friend, 30F, who recently confided in me about her past infidelity. Her partner, 32M, is under the impression they've always been faithful. I'm conflicted about whether to encourage her to come clean or stay out of it. I value both their friendships and fear the fallout. Any advice? #WhatWouldYouDo #FriendshipDilemma #Honesty