what would she want compensation for? its not like the optician gave her glaucoma and cataracts, and apart from the migraines what has she materially lost? Has the doctor at the hospital said that the disease has become untreatable from being left for 3 months?
Myself personally i would complain about the optician, cause its hardly likely these problems occurred in the last 3 months. However your gran is a grown up and perfectly able to state whether she wants to take it further or not