Hey, Voy. Sorry to hear about your cat. Stay positive; nothing aids healing better than a positive attitude. Firstly, let me say that I know very little about cats. I'm more of a dog person, I'm afraid... HOWEVER, I found this link, for a medicine called
Peritan FP, and it's got some macela leaf in it, which I'm familiar with as a powerful anti-inflammatory, antiviral, liver tonic, and an herb that I'm unfamiliar with, called cat's claw (fitting, eh?) that is also used as an anti-inflammatory. Both are traditional South American herbs, and from the little I've read on FIP, it causes all kinds of nasty inflammation, and potentially liver shutdown, so if you're into taking a shot at it, these herbs could give your cat a chance. Try probiotic foods as well, since your cat's guts are likely out of whack as well from the body's attempts to curb the virus; lots of fresh water, sunlight, and limited (no) exposure to potential chemical contaminants will all add to chances that your cat will survive. I'll send pos vibes your way. Good Luck!