Usually by the time the vet is able to pinpoint FIP as the cause of your cat's condition, the cat is pretty far along. So long as your cat is in pretty good shape, not in any pain or discomfort, there is no reason to euthanize it. Even if your cat is happy and healthy, however, you MUST make sure you keep it indoors and away from other cats. If you feel that this will be too great a compromise on its quality of life, it is better to euthanize it. Since the cause of transmission is not known, by allowing your FIP+ cat outside, you could cause numerous other cats to become ill, and even further spread the disease. But please keep your cat's welfare foremost in your mind. When its systems begin to fail, when it is in obvious discomfort, you are only making things worse by delaying the inevitable. Keep him or her as happy and as comfortable for as long as possible, that is unfortunately the only solution at this point.