Did the vet mention she was under weight?
Young birds with this often have secondary bacterial infections as their immune system is overwhelmed and not fully developed. To me she looks like she feels unwell. Did vet feel she had a secondary infection? If not you might ask if this is a possibility and treatment with antibiotic might make her feel better. While it wont treat PBFD , they are often prone to other infection.
Unfortunately animals with dilute colors are often linked genetically with lower or deficient immune systems.
Supportive supplemental warmth is always helpful to ill birds. I like the sweeter heater a radiant heat panel but you can get one made fir little baby chickens sold at farm and feed stores cheaper. Make sure to place off to one side with a perch by it , but so she can also move away and enjoy the rest of cage without if she doesn't want at the time.
I think offering her supplemental baby bird formula made a little thicker like Greek yogurt thickness, offer in a dish or from finger. Once a day would be supportive. Goes bad quickly so remove after 20 mins it so and don't make it up ahead and store in fridge. Inky a little fresh when you offer. It's easy to digest and will help keep weight on her. It's not a must , but definitely something I would do.
If you can have her eating healthy that also support the body , sprouts, warm cooked sweet potato, bell pepper leafy greens s d other veggies.
I'm really sorry for both of you. What a heart breaker. I'm so glad you took her to the veterinarian and are providing the best care and love for her.
