Looking for advice about sick hen with diarrhea and is lethargic

Hello,

Any advice would be greatly appreciated. I have a 15 month old buff orphington who became a bit secluded about a week ago from the rest of the flock. I lifted her up on to the roost one night but didn't think anything about it. The next day I went in with a banana treat and she didn't even care. I have her in the basement since last Thursday. When I brought her in, her vent feathers were all matted with fecal matter. I gave her a bath and let her soak and dissolved epsom salts thinking it was vent gleet. She sits with her eyes mostly shut, is feathered up like she is cold even though the basement has a wood stove in it, and I have been feeding her water since then with a syringe.
She will occasionally eat food, but mostly just lays on it. By food I mean raisins, white bread, and oats. She will not touch her layer feed. When I bring her out to the coop, she drinks and drinks. But inside she will not drink willingly. I read about coccidosis, and bought Corid. She has not responded to that either. She sits in her feces and won't move all day, which is very mucusy and yellow with some green matter, even though she has not consumed anything green in a while. No worms present. I did check to see if she was egg bound, and she is not.

If anyone has any advice I would most greatful. Thank you,
Alicia