Hello! We are in desperate need of advice to help reduce the fur shedding of my mums cat, if anyone can please advise!

My mum was delivered "puss wuss" by the cat distribution network a few years back (2019ish), in recent years her shedding has got worse, to the point furballs are being coughed up several times a week and she only has to sit on your lap and you get the results in the photo I've attached, the rate of shed doesn't change by season, she sheds terribly 12 months of the year. My mum has tried changing her food but if she doesn't like it, she will starve herself, we also tried an additive to her food that was supposed to help but it did nothing. We groom her and get balls the size of ping pong balls from each brush stroke and its never ending, literally we've never got to a point where the brush strokes stop producing ping pong ball sized hairballs.

We are reluctant to keep trying different additives without recommendation and food changes are extremely difficult. Can anyone recommend a specific food, allergen, additive, vitamin, etc...to introduce or avoid for her specific breed (tortoise shell) and assumed age. We don't know her exact age but she's starting to slow down (but does get the zoomies now and then) and claw retraction is becoming slower so she is older. She was a stray in my mums garden shed for 12-18 months before she approached the house and came inside around 2019, so at least 10 years old.

Extreme shedding, PLEASE HELP!