Try using natural cat litter such as pine. You do not have to scoop. When the pine crumbles into powder, you throw it out and put new litter in. I, myself, was getting a lot of sinus problems with scooping cat litter. I started using the pine, and I do not have any more sinus problems. The litter was too strong for me, and I can only imagine how strong it was for the cats.
Welcome to the forum!
I have to tell you, Zodiacqueen is right. This is not a condition that can go untreated and there really is no option but to take the cat to a vet as soon as you can. Actually, I can't quite tell from your question if you have been to the vet already. Since you know that the cat has feline respiratory disease, have you already had a diagnosis and a prescription for antibiotics?
Once you have done that, there are natural supplements and dietary regimens that can help a lot. You can ask your vet about it or you could come back and ask us about the specifics.
Let us know what happens, ok?
This condition is a bacterial infection that will not go away without antibiotics. Cats can die from this. So, if your brother wants the cat to live he needs to get the cat to the vet without delay.
There's plenty of information on the internet about this condition. You'll see that this condition is not to be taken lightly.