Bonus points for it being based in her Christianity, rather than in defiance of it.
Joyce: Jesus says to love your neighbor as yourself. Luke 10:27. That's everyone. Not just people we like. Not everyone except the Samaritans. Everyone. And so I will not act likeŚlike a jerk to my best friend and her family. Anyway, that's what the Son of God says. He is my higher authority, my moral foundation. Not you.
Dorothy figuring out Amazi-Girl's identity just by putting the evidence together is relatively impressive considering that so few others in canon saw through the Clark Kenting (Amber's roommate Dina may be the only other one to figure it out without unmasking her). The real CMOA, though, is when she decides not to expose Amber after all, and her explanation is funny and a bit heartwarming to boot.