That is a lot of human food for one cat.
Canned fish is missing several key nutrients that cats need - minerals and saturated fats for example. The liver and chicken are also low in minerals and saturated fat, little taurine in those especially if they are cooked. The advice for humans is treats to comprise no more than 10% of total calories - all the human food I would class as treats for a cat.
Fish is not natural prey for a cat and over the long term too much - especially tuna - can cause steatitis which is a painful inflammatory disease, as arthritis can also be. Weight loss is also a symptom of deficiencies.
Not diagnosing as I am not a veterinarian just wondering if the dietary imbalances might be contributing to the aging process. Does the vet know quite how much human fish the cat is eating or your mum underplaying it?
For weight gain cats need complete animal protein and
saturated fats, with a balance of vitamins and minerals to help the body digest/ metabolise those nutrients and build/ repair cells. Is it possible your mum could change her cat over to a fish-based complete cat food?