It is tough to say what it is without proper tests to confirm. There are so many respiratory illnesses that look similar, it would be a guess at best. Could be MS, MG, ILT, infectious bronchitis, CRD, etc. This is the hardest part of raising chickens.
The watery stools are typical when they are not eating enough. Have your tried alternative foods...things she might like to get something in her for strength. Scrambled egg, hard boiled eggs, yogurt, oatmeal..are all good choices. The warmth will help her keep her strength to fight whatever is going on, so I would keep her as comfortable as possible (not too hot and not too cool).
How does her belly feel? If you go in between her legs from the rear, do you feel any swelling? If so, is it hard or mushy soft? I'm still thinking something else may be going on that is breaking her system down to allow for viral symptoms.
Jody