I know time is of the essence, can you get her poo sample examined by a veterinarian?
Her symptoms sound similar to this:
https://www.thepoultrysite.com/disease-guide/salmonella-pullorum-pullorum-disease-bacillary-white-diarrhoea
And she needs antibiotics if that is what it is...
I also wonder...
What does her poo look like? Is her butt clean? How many chickens do you have and how old are they? Is the rooster still sneezing? PIctures of both the hen and the rooster? What type of bedding do you use? And what is your location? so we can know what kind of weather conditions.
@Eggcessive