Hi.
I'm so sorry for your hen...!
I don't know if I could be of help, but in the case you are really desesperate...
...Some years ago, I have cured a Gournay hen very much weakened and literally dying from Salpingitis.
I don't remember all the exact details, but at the time, she laid really disgusting waste...
I knew some things then, and after some more research, I sadly had to conclude my hen had Salpingintis...
Problem was : I did not know any medication to treat Salpingitis!!
Buuuuuut... since Salpingitis in chickens is renowed to be (often) due to E. coli, my priority was not to treat Salpingintis since it seemed like a lost cause : instead, I choose to treat E. coli... in the hope, of course, it would save my hen!
And my choice was probably for the better, for resulting since then in : a hen alive, healthy, happy... laying eggs again - some of them being double-yolked HUGE eggs!
So... what I did to heal my hen (noting that's was technically an experience, since that was a first for me) :
Everyday, I gave her herbs and ground spices - especially cumin, which is told to help against E. coli!
(I was not even afraid to give her too much of these, since she was dying anyway...)
Herbs = Herbes de Provence + A LOT OF thyme
Spices = Bell pepper, Cayenne pepper, Espelette pepper, Ginger, Curcumin, Cumin
Also, I added a lot of garlic (powder)!
I mixed all these herbs and spice with (natural) yogurt, wet cat food, OR - occasionally - a mix of milk and bread... and I added a bit of honey in the mix! Sometimes, I also put Olive Oil in, and a little bit of cinnamon!
(...I think I also put some liquid Calcium in the mix...?)
In addition of her usual food, I gave her that mix to eat for a long time (don't remember how much exactly, but was loooooooong), hoping she would get better.
And sometimes, I also put Apple Cider Vinegar in a plastic waterer, thinking it would help her!
And certainly thanks to that, my hen survived!!
Took time, but she finally got back into shape, and since then, she has even been laying some massive eggs (what she never did before)!!
Never again has she been sick...!!
...Don't know if it could help you...
I don't even know what exactly cured my hen! (Cumin, yes... but for the other things I put in the mix I feed my Gournay? Not sure. I just tried what I could, by using what is renowned good for the immune system!)
But if your hen is dying, what do you have to lose by trying to feed her how I did?
Maybe you hen will die, but maybe you can save her...?
Good luck.