What does her poop look like? Photos?
Have you treated for Coccidiosis? If you haven't, I would consider doing so, just by looking at her condition.
Have you ever noticed any coughing, sneezing, watery or bubbly eyes with her or have you ever had respiratory illness within your flock? Her eye looks like it may be a bit puffy. I'd keep watch on that and make sure the coop/housing has adequate ventilation and that droppings are kept under control to help prevent ammonia in the housing.
Liquid Corid dose is 2tsp or Powdered Corid Dose is 1 1/2tsp per gallon of water given for 5-7 days as the only source of drinking water.
Do not add any extra vitamins/electrolytes that contain B1(Thiamine) to food or water during the course of treatment.