How would a vet treat a feline with conjestive heart failure?

My sister had this cat for 14 years. She took her to the vet because the cat was breathing funny. The doctor told her there was fluid in the cats lungs nothing else. He treated her with Lasix. My sister begged the vet to stablize the cat before she brung her home. The cat was not eating or taking water when she got home. My sister gave her water with an eye dropper. The vet did not tell my sister the cat had conjestive heart failure. He just said water in the pleural cavity surrounding the lung. This cat was an indoor cat and very spoiled. She was rather heavy and my sister always fed her the best food she could afford. Anyway, my sister found out what the cat had after rushing her in to the after hours vet and he recommended the cat be put to sleep. My sister was shocked because she got the impression the cat could be treated. Was the vet neglectful in not telling her what the cat was diagnosed with in the first place? Why did he not keep the cat until she was stable?