I absolutely get the logic she had going in, the issue is she never stopped to think about how her friend actually felt. And in fairness her friend never expressed a word of it. From jiah’s perspective it’s a win win, she gets to help her friend bring her novel that she loves and her friend works so hard on to life! That’s collaboration baby! Friendship for the win!
But since she’d never been in that place before she had no idea how harshly jealousy can sting, and when someone who it feels like has been handed win after win with no effort casually says “here I fixed this thing you’ve been pouring your heart and soul into” that green flicker explodes into a raging fire like pouring water in a grease fire.
I don’t fault Jiah here, she was young and dumb, her heart was squarely in the right place. And she presumably realizes how cruel she had unknowingly been which I’m guessing is why she goes ghost with everyone. And something clearly clicks with her friend after this since she becomes insanely successful, so really her action WAS the right thing to do. It was just also accidentally hurtful in a way only someone you cherish deeply could do.