First, you need to know what is actually wrong with your mother. Did she develop a cataract? Why does the doctor think she has something wrong with her retina? What kind of scan? Why would he be concerned about her eye muscles, when this doesn't sound like an eye muscle problem? You need a better understanding of her situation.
Second, glucosamine is a large molecule that does not get inside the eye and thus is very unlikely to be the culprit. Further, when something you take orally affects you vision, it usually affects both eyes.
I neglected to mention that my mom does have the start of a cataract. The doctor noticed it last year and said it was very very small and the size has not changed. He checked her macula for damage and did an OCT scan. He is not sure at this point why her vision in her left eye has changed so much and I think that is why he is suggesting a retina specialist.
He said the cataract being very small is not likely to be the reason for vision loss. I don't know how big the cataract is but being small can it cause this much vision loss?
I apologize for not explaining properly.