That question would have to be answered by a vet examining her, if you can find one who treats chickens. However...
My opinion, from what you've explained, is that she is very bad with peritonitis and ascites due to a deep internal infection, and that at her age, there is little hope. I'm sorry, but at this stage and age that is the usual outcome. She likely has ovarian cancer, or something.
It honestly may be kinder to put her out of her misery.
When the time is right (and I think it is soon, but you will have to decide that), I can recommend the CO2 method with dry ice. It's bloodless, quick and painless for the bird, and recommended kill method by the American Veterinarian Association (my daughter used to be a vet tech).
Get a 5 gallon bucket with lid (like at Home Depot or cat litter bucket). Get about 1/2 lb of dry ice (from your local grocer...I get mine at Kroegers...it's pennies cheap... whole thing should be about $2). Put dry ice at bottom of bucket. Pour warm water. Place lid, with a small side of lid vented. Let BIG cloud form (key to success and instant kill). You can put a plastic coffee lid or something on the dry ice as a kindness so the bird doesn't rest on it. Place bird in bucket. Close lid, again with a small side vented. You will hear a quick gasp. The bird will be rendered immediately unconscious (same as the CO2 drop systems in office buildings...a gasp and you're out). You will hear fluttering for about 30 to 45 seconds, then silence. The bird asphyxiates while unconscious during that time. The body may jerk a time or two afterward, but the bird is well dead. (You can check eye pupil response.) To be safe, I wait at least 3 to 5 minutes.
It's how I kill my sweethearts when it is time and I want a gentle out.
Hopefully I'm wrong...but those symptoms, t
Is her abdomen hard or feels like fluid?
Likely EYP, cancer or some reproductive disorder is the cause of fluid (if there's fluid). Draining may buy her a few more weeks or months if that's something you want to consider doing. It's not a cure by any means, but a measure of supportive care that some folks extend to their hens. Draining does have it's risks like causing infection from the needle stick or draining too much fluid too fast (causing cardiac arrest) are a couple of things that could happen, so you will want to consider those too.
Aloe Detox is another method some folks use. Again, it's a supportive care measure and sometimes can manage fluid for a good while.
@coach723 gives details in this
POST.
I'm very sorry about your hen. It really stinks to watch them with reproductive problems. When I have one that is in a slow decline, I access them daily for indications that it's time to let them go.
Thank you