Ahhhh, what a shame! I am so sorry, my dear. It's always terrible news when we find out one of our pets has a real bad disease for which we cannot do anything about... But, the good news is that she obviously has the chronic form of it and not the acute. On another note, you should have your gray tested for it and, if the gray tests negative, keep them completely separate from each other. And I do mean 'completely' so no sharing dishes, rooms or anything - this virus is VERY difficult to contain because you can't kill it and stays alive and dormant in the environment for up to a year so quarantine needs to be super strict meaning washing hands with disinfectant every time you handle the IRN, wearing protective clothing that should not leave the room where the IRN is quarantined (like a large housedress that you can leave hanging on the door), etc. Also get yourself some Virkon S which, as far as I know, is the only disinfectant that kind of works on it (it doesn't kill the virus but it deactivates it). This disease is fatal, HIGHLY contagious and there is no cure so if your gray is not already infected, you need to take the outmost care.
Unfortunately and as I am sure your vet already explained, there is no cure or even a treatment for this disease and, once it affects the beak and there is lethargy, I suggest you euthanize because you do not want the poor bird suffering for months before it dies on its own. The most important thing to do now is to watch her very carefully for any symptoms of a secondary infection because with their immune system so compromised, once you notice something, you need to treat asap because they go down super fast so ask your vet if he would be willing to keep you supplied with a bit of a wide spectrum antibiotic so you can keep it handy, just in case. He might say you could always bring the bird back whenever you notice or suspect something but there is nothing worse for a sick bird than to be stressed out and taking them to the vet is VERY stressful to them so the 'usual' solution is not really a viable one for a very sick bird.
Now, let's go into what you can do for her - namely, a good quality of life and supportive treatment so as to keep her immune system working as best it can:
1) Keep her very warm (and this means really warm, around 85-90 degrees): either with a heating pad under her cage (put a thermometer halfway up her cage and make sure it stays around 85) or a safe space heater in the room where she is kept (do not use lamps, they are not good for this).
2) Feed her the right diet: absolutely no free-feeding protein or any dry food (you need to keep her 'filters', the liver and kidneys, in perfect working order), lots and lots of fresh fruits (for the phytonutrients that 'feed' her gut flora where 70% of the immune system is located), especially blue and purple ones that are the highest in antioxidants and yellow and orange for the betacarotene and C vitamin.
3) Supplement her with a good quality multivitamin/mineral - use a powder that dissolves in water (not the already liquid drops or the powder that you sprinkle on the food because the first is already too degraded and the second does not allow for any acurate dosage), take the 'old' water at night when she falls asleep and replace it with freshly medicated one early in the am -she will go straight for it to drink and will get the most benefit out of the supplement. But be careful not to over supplement - use a daily dose 3 times a week when she is already eating a large, varied range of veggies and fruits.
4) Supplement her with immune system booster like oregano and the tiniest bit of garlic (mine get it once a week mixed with their gloop and they love it), echinacea (but do it for five days and rest for two for best performance), elderberry, ginger (again, mine get it in their gloop and they love it) and some ginseng.
I remember reading an article some time ago about an experimental anti-virus drug called Act or something like that for circovirus treatment in mammals that was derived from the greater burdock plant (used in Chinese Herbal medicine for thousands of years) so I did a bit of research for you and here are two links for you to read:
https://www.sciencedirect.com/science/a ... 8815300837https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3073957/Good luck and let us know how things go with your gray and little IRN.