I know you said you tried tuna before, but how about what I call "Tuna Soup"? It used to work when I had kitties that wouldn't eat. Take some chunk light tuna packed in water and mash it up real good (don't leave any chunks) and add enough warm water to make it the consistance of gravy - not too thick yet not too thin. It might be worth a try since she doesn't seem to want anything else. Good luck!
Come on Anika - please eat for your Mom!