Looks like dropsy - I can see the scales sticking out.
If she's still eating, you could try buying or mixing up a batch of medicated food with some crushed garlic as an appetite enhancer.
If she's not eating, it would probably be better to end her life humanely.
I find that the most humane methods of wuthanizing a fish are also the most difficult for us. Decapitation with a sharp knife, or crushing the head with a blunt object kills instantly and painlessly. If you can overcome the squeamishness, it's the best way,
IME.