IFN omega (Virbagen Omega) was initially given subcutaneously at 1 MU/kg every other day, and then once every week for variable period if remission was seen.
Glucocorticoid: (dexamethasone 1 mg/kg intrathoracic or intraperitoneal injection once only) or prednisolone. Oral prednisolone was initially given at 2 mg/kg once daily, and the dosage was gradually tapered to 0.5 mg/kg every other day after remission.In the study, 4 cats of 12 completely recovered and two survived 4 and 5 months. Those cats which recovered completely all had the effusive form of FIP and were relatively older cats.
Low sample size but a statistically strongly significant result for a disease that is, otherwise, 99% deadly.
Moreover, treatment is not that invasive and, since the cat is currently eating well, gaining weigth, being lively and performing all her bodily function correcly, it may be worth attempting (if available in my country being the United Arab Emirates)