It really is an unfair disease. Willow was almost 9 years old when I lost her to it last month. She had been healthy her whole life, lived indoors only, and had minimal exposure to other cats--only a few fellow feline housemates over the years. Odo had been with us for more than a year when she was tentatively diagnosed with dry FIP because of her ultrasound results and her high protein and lower albumin/globulin ratio. She was with me for a little over a month after her blood work came back with abnormalities. We did everything we could to keep her comfortable, including putting in a feeding tube so she would get adequate nutrition. The vets didn't think middle aged cats got FIP, so they were not sure about her diagnosis until it progressed. FIP was eventually confirmed via tissues samples sent to Auburn University after her passing.
It is rare that a cat survives FIP, unfortunately. A few people have found that Feline Interferon has helped their cats, but getting it is tricky in the United States as it is not approved here and has to be imported. Also, it's very expensive, and the research that has been done is very limited and inconclusive.
What tests were done to diagnose your kitty? The only "conclusive" tests are done through PCR on tissue samples. Since most of these kitties are too sick for surgery, the biopsies must be done after they have passed on. If the vet's diagnosis is based solely on a Feline Enteric Coronavirus or FIP titer test, get a second opinion from another vet. Ask the vet to explain to you how he/she came to the conclusion that your kitty has FIP. Enlarged kidneys alone might be managable for a while, but if it is indeed FIP, I would suggest talking with your vet about your kitty's quality of life and how to know when it's time to let go.