My elderly neighbour has a 12 yr old greyhound who is extremely

I can understand the being stubborn. Sometimes the being stubborn is also due to not wanting to possibly know what is going on as being in the dark for them seems better. Thank you for taking the time and consideration to help him and his dog out. At 12 years of age for a greyhound, that is definitely getting up there. You are correct in that I was questioning diabetes as that would be a potential issue for weight loss and a great appetite associated with excess drinking or urination. To screen for this simply, you could catch some urine and take that to the vet to have them screen for glucose in the urine. If it is there, you have your answer. If it isn't there, then time to move to something else. Could there be an underlying cancer that is robbing the body of nutrition? This has to be considered. Could it be liver or renal disease? Also possible but usually they have the appetite progressively deteriorate. With elderly owners, I have to bring up the nutrition as sometimes they may not be feeding as regularly at home as they may be forgetting things like this. I don't want you to offend this neighbor, please just let him know that you want to help him be with his dog for as long as possible. Your goal is to help his buddy out. If the vet can come to the house and check him out, that would be great as they could do a good physical evaluation and draw some blood for blood work.