Does she have any yellow lesions or canker inside the beak?
I wonder if a flush would be beneficial since it's so rancid smelling. It's possible that it's just a really bad yeast infection since it's been going for a while. Yogurt generally will not take care of sour crop. @azygous...
Is the skin discolored under the feathers?
Smelling bad and pendulous, I'd treat as Sour Crop according to the article linked below.
https://www.backyardchickens.com/articles/how-to-treat-sour-crop-and-impacted-crop-and-how-to-know-which-one-youre-dealing-with.73607/