I had a girl, she constantly gave me soft shell eggs.
I noticed she looked like a bowling ball, with 2 little legs holding her up.
She was a special girl. I thought it was EYP, so I brought her to the vet.
What it really was, she had cancer, and her liver and kidneys couldn't process her fluid waste.
Her body, instead of expelling them, just held onto the waste, and she blew up like a balloon.
The vet drained her abdomen every week, for about 2 months, until she just gave out.
I had another hen, same problem, but not special.
I drained her myself a few times, but she was too far gone.
She popped like a balloon when I tried to autopsy her, and a nasty urine looking fluid gushed out.
If you want to try it, take a coarse needle syringe, and punch a hole just into the skin, no further, into her abdomen.
Kind of like where her thigh rubs her belly.
If this is the cause, her abdomen will be bloated, and this will give her some relief.
Chickens are very prone to cancers, unfortunately, since they are chickens, there is no research or cure.
Warning.... I am not telling you how to do it, just telling you what I have been through.
I will probably get yelled at from somebody, but the chioce is yours.
Good luck with her.
Roberta