Hi Mike,
FIP is a tough condition to treat, but lately, through Dr. Jodi Van Tine, we've seen some successes from taking a TB obstruction approach. In the Chinese medical model, water is supposed to descend through all three burners down to the bladder, under the guidance of descending Lung Qi. The Triple Burner is the corridor where this descent occurs. So an obstruction here produces ascites, pleural effusions and the like. The obstruction occurs in the middle burner, producing hepatomegaly, possibly splenomegaly, and a purplish tongue. Lung Qi also has problems descending, leading to the cough. The TB channel ends at the eye. Eye redness is thus not uncommon in TB cases.
Xiao Chai Hu Tang is well suited to these cases. It's not much of a diuretic, but it has a complex immune modulating effect, without being either purely suppressive or purely tonifying. It addresses very well extreme tracheal sensitivity and is a smooth muscle dilator. There are literally dozens of studies looking at its benefits on liver pathology. So, all in all, it's a good choice for this patient, at least to start. Down the road, a pure Kidney deficiency picture eventually appears, calling for something like Rehmannia Eight. But I wouldn't introduce tonics yet until you've opened the TB up.
Good luck. It could be a really fun case for you, and I'm sure followers of the forum here will be interested to see how it goes.
All the best,
Steve