This is only my opinion Setter but if the cat is very old , I would not risk unless they had really good information and back up.
Sometimes it is better to let an old cat live out its life without trying to force it to eat or take stuff that it doesnt like, it will just get stressed and possibly more ill.
if the cat still has a quality of life, then its time has not come but if it becomes in pain or too sick to eat or drink and a good vet cant help, it would be then be time for it to be allowed to pass on peacefully.