The vet that we used for an at-home euthanasia for one of our cats has a "Quality of Life Assessment" on their website. It's a list of normal activities for pets, and how owners can judge whether the pet's quality of life is impacted. Inability to control elimination is one of the factors. You might share this with your friend.

https://peacefulpassage.net/end-of-life-resources/

My three grandkids will be losing a beloved dog tomorrow. In January, she developed a tumor on the inside of her muzzle that turned out to be inoperable. She's now in terrible shape, down to skin and bones, weak and disoriented, her eyes running, drooling uncontrollably, and emitting a terrible stench when she breathes. She'll be euthanised at home tomorrow. It's been a tough decision for my son, their dad. He rescued her from the street 10 years ago. The kids have been through the deaths of two cats and a dog (they're 11, 8, and 3). It's so sad to see the dog going through this decline. She still eats and drinks a bit but doesn't play anymore. Falls down sometimes. She always slept in the kids' bedrooms, but can't get up the stairs anymore.

and I understand that she's close to dying -- just not there yet.

And my friend doesn't have a lot of money to spend on exploratory tests. And if they found out she had cancer, for example, there would be little point in treating a cat that old. So why do the testing if she wouldn't be going forward with treatments?

Please have her take the cat to a Vet ASAP
Could be very treatable