I have a hen who suffers from Ascites (fluid build up) and I use a needle/syringe to drain her every two to three months. If not, she will fill so badly that she cannot walk and would die. But after draining, she is good to go for another two to three months and is normal in every other way and continues to lay large brown eggs. Just insert needle in water balloon "belly" and withdraw fluid, I go in near the bottom, under the vent. If clear or slightly colored water comes out, it's Ascites and you can continue inserting needle and draining. Some of the holes will continue to leak and drain on their own. If you use a needle with a large syringe you won't have to reinsert so many times. My hen shows no discomfort whatsoever when I do this. Just feel the water balloon and you can feel it's nothing but skin and fluid-filled bag - insert anywhere in bag.
If you can't get any fluid out, it could be that she's an internal layer and it's yolk material that is building up and cooking inside her - unfortunately, if that's the case, there's not much you can do.
Many others have also drained their hen and had good results. It only fixes the problem and is not a cure so it will have to be repeated. She will be so much more comfortable and back to normal once relieved of the weight and pressure of all that fluid.