IS the fluid IN her lungs or AROUND her lungs (in the chest cavity)? That will make a big difference in what disease is actually to blame, and what medications can be used.
Fluid IN the lungs is usually due to heart disease - congestive heart failure.
Fluid AROUND the lungs (pleural effusion) is often due to cancer in the chest, in a cat, lymphoma is the most common cause. Other sources of fluid around the lungs are FIP (an immune-mediated viral disease), pyothorax (pus from an infection), chylothorax (milky lymphatic fluid), or blood from trauma or cancer.
WIth cancer in the chest, the prognosis is, as one might expect, not great -- however, I have seen some patients go many months with medication and occasional to frequent draining of the fluid. A fluid analysis would really be very helpful.
Prednisone is typically a good choice when there's fluid around the lungs, as long as it's not pus (pyothorax), because prednisone can help with inflammation or some cancers (lymphoma). It may shrink a tumor and slow down fluid production by that tumor, as well as helping the feeling of well-being.
Good Luck, let me know what you find out.