I would try again with a visiting optician. Some of them specialise in patients who are incapable of co-operation (such as following instructions).
There are tests such as direct examination of the eyes which can look for degenerative conditions. The only co-operation required is to keep the eyes open and let the optician look in them.
Unfortunately the more definitive tests require that the patient is at least capable of putting their head in a specific position, looking straight ahead, keeping still etc.
The test that checks for high pressure in the eyes is a simple device that uses a puff of air. High pressure can lead to glaucoma. But the test that checks to see if there is any actual vision loss requires that the patient press a button when they see lights.
Unfortunately tests to check what a patient can see require that the patient is able to tell you if they can or can't!
The standard eye-chart test for spectacles can sometimes be replaced by using a device that directly measures the eyes . The result is not so accurate because vision is partially subjective, which is why the optician asks you "better, or worse?" so many times. If your mum's worsening vision is simply the normal experience dwith aging, this may be an option.
If your mum's vision is deteriorating, espescially if this is rapid, then his eyes need to be examined because this could be a sign of macular degeneration. There are two sorts - "dry" AMD which happens slowly and gradually, and "wet" which can be much more sudden and rapid. Both show physical signs in the eyes that will be seen by a doctor/optician on examination. Definitive diagnosis requires a test done is hospital that uses a special dye and a camera to image the back of the eye but this requires the patient to be able to tolerate an injection (in a vein on the back of the hand) and to sit still looking straight ahead into the camera. Dry AMD is untreatable, but wet AMD is (for some people)
The typical signs of AMD are increasingly poor vision, poor colour vision, a need for a brighter light, etc. When advanced it leads to a blind spot in the centre vision, this is th epart of the eye that is used for detailed vision so things like reading or recognising faces becomes very difficult. Wet AMD typically leads to distortions in vision, particularly straight lines as well as the other symptoms. It is very important to get an examination, since wet AMD can progress very rapidly (sometimes in weeks or months) and the treatment cannot restore any vision that is already lost.
If you cannot find an optician that can help, see your GP. They would also be able to conduct a basic physical exam of the eyes and make a hospital referral if necessary.