Poor dear, she does have a distended abdomen

Can you tell if it feels like fluid or ?
It's possible she may have tumor(s), cancer or is suffering from common reproductive issues like Peritonitis, Salpingitis, etc.
If there's fluid it can sometimes be drained to give the hen some relief for a short period of time. Some folks have found that
offering a detox can be helpful for a time, others give an antibiotic to see if that helps for a while. Of course these are supportive measure to make the hen more comfortable and sometimes they can live for a good while longer. Other measures you can take would be to make sure her crop is emptying overnight (crop issues seem to go hand-in-hand with repro problems), that she's lice/mite free and that she doesn't get overloaded with worms.