chapter 45 why does shay know about tallys secrect spy mission?

Tally is devastated to witness the brain damage the operation has wrought on her best friend. Shay is untroubled and upbeat, completely and utterly changed. She lives in New Pretty Town now, but has been brought in by Dr. Cable to talk to the caged Smokies. David goes to break them out, and Tally and Shay stay behind. A carefree Shay assures Tally that she forgives her, but Tally knows it is only the lesions talking. While David and Tally help the prisoners out of their cages, Shay buys time by telling the Specials trying to contact Dr. Cable through her tablet that she's busy. David and Tally manage to gather everyone, but Az is conspicuously missing. Maddy tells him that he can’t help his father; Az is dead.