I take care of an elderly friends cat's litter tray for him (it gets emptied and cleaned each evening). When I arrive to do it there's an overwhelming aroma of ammonia in the room where the tray is kept. Ive tried the obvious like encouraging the cat to drink more water and she (the cat) is healthy. Any suggestions on how the ammonia in her urine can be weakened or the smell of ammonia be masked?
if the cat is old too it may be an idea to get a vet to check out its kidneys. When the kidneys are not working as well as they should be the urine smells as you describe.
Try adding more litter to help absob all of the urine. Encourage her to drink. MAke sure she isn't dehydrated and besides that, there really is not a lot that can be done. Check the room for any "accidents" as well, just in case they are adding to the smell.