Sounds good. I think the explanation for the weight gain will be really important. You could go for a supernatural angle by having her be cursed or something, which would be simple, but ideally I think the method needs to be integral and something you spend the book waiting to learn. Maybe the fat friend sabotages her with a mad scientist style virus to teach her a lesson and then the betrayal is learnt near the end for the big twist. If we never learn why she gains weight, the story might feel empty. In general, I see a lot of potential. It should be a very emotional book filled with the main character going through depression, desperation, and lashing-out. On a personal note, I'm a fan of sad endings. There is a temptation to have a betrayal revealed and then have vengeance done before returning to normal, but I will personally really enjoy the slow, miserable death. I would love a real tragedy. Plus, it will leave the reader despising fat and then project that onto regular fats. I'm with you on that. Oh! Maybe the fat friend saboteur could happen to suffer comparable obesity complications at the same time and gradually comes to realise the horrible mistakes of he life and what a horrible thing she has done. Double miserable ending!