Sorry this happened, JosienBB. It does happen to the best of us, unfortunately - accidents happen in a multi-parrot home even when we are absolutely vigilant. The walnuts are fatty, so keep up with those, and anything else she shows an interest in - you're just trying to put weight on in the short-term so if it's a little unhealthy, so be it. When my elderly 'zon was very ill, he was only interested in terrible foods - cheese, corn, scrambled eggs, chips - my vet encouraged me to just keep him eating, whatever it took.
I notice you are in Ontario - not sure what part of the province you're from, but so am I - if there is any way I can help, please PM me.