Good evening, Amanda. I am so very sorry to hear that your little girl has a case of suspected FIP.
Her symptoms do fit with "wet" or effusive FIP, although it seems no two cats ever have the same symptoms. It can take weeks to months for a case FIP to develop, so she likely came to you guys with the precursors needed to develop this affliction.
I'm not sure how much you guys know about FIP, but it's usually the very young who develop it. We know that FIP can develop from the corona virus. There are around 60 types of corona virus, but not all of them cause FIP. When a cat does have the "right" type of corona virus, that virus must then mutate into the virus that subsequently causes the symptoms of FIP. It's uncommon for this to happen, but it's debilitating and eventually fatal for the cat. There are cases of a litter of kittens all being exposed and only one of the kittens developing FIP. In this, it would be very unlikely for your 10 year old cat to develop FIP. Again, she'd not only need to be exposed, but then the virus would need to mutate. Often, the more stable immune systems of adults keep this from happening. Alas, kittens just don't have the stable immune system that an adult does, which is why it can be so detrimental.
There's more information here: http://www.vet.cornell.edu/fhc/health_information/brochure_ftp.cfm
While veterinarians and researchers are working together to find a solution and treatment for FIP, we have nothing available at this time. One of the most selfless things that we can do, however, is donate a DNA sample from our afflicted companions to further research to eventually benefit future generations. There's more information here on donating samples, current research and more information on FIP: http://sockfip.org/
As you're already doing, the best care is supportive care. In general, once cats begin having issues breathing...it's time to make that final decision. Removing fluid from the abdomen can buy us time, but it does not fix the problem. Most often, once the abdominal swelling has reached a certain point, lethargy sets in, the pet no longer wishes to eat or drink and the kind thing is to allow them a pain-free passing.
If my answer has satisfied your question, please take the time to issue positive feedback. This is how experts are compensated for their time with each customer, like yourself. If questions remain, please reply and I will be happy to help you further.