Water belly generally is not contagious. A couple of common causes of fluid to build up in the abdomen are reproductive issues (cancer, peritonitis, Salpingitis, etc.) or organ dysfunction (heart failure, fatty liver disease, etc.).
If there is fluid in the abdomen, sometimes it can be drained to give the hen some relief for a short period of time There are risks to doing this - infection can be introduced, hens can go into shock and die. Possible there may not be fluid, but a mass or tumor instead.
If it's fluid, it can build back up, and of course whatever is causing it, is still there.
A detox may also help too.
https://www.backyardchickens.com/th...-breath-if-you-hold-her.1284253/post-20750855
Sometimes a hen can live for a while longer when offered supportive care while others decline. No one can tell you how long she can live.
I'm very sorry that she's not doing well. I really stinks when a hen is sick.