Strange because the guide said that donna is needed at max weight or just as copy paste:
Can be recruited once Donna is at max size
Victoria has one ending
Visit Victoria at the donut shop → Treat yourself and Victoria → repeat until cooking event happens → Return the next day → View the cinnamon swirlies event → Return the next day → Speak with Donna → Visit the park and go to the Ice cream stand → Speak with Donna → Visit the Gym once the health bar has begun selling milkshakes → Speak with Donna → Visit the Buffet → Give her the Sandwich
though i haven't tried to unlock her yet so i don't really know what might be wrong.