Yes, diabetes can cause some eye damages. In common, people may suffered from retina damages if they have diabetes. Some people called this damages as diabetic retinopathy. And the diabetic retinopathy is just a process. If you ignore it, it is possible to cause total loss of vision. That is to say, diabetic retinopathy can result in blindness. So, you'd better take your grandma to see an doctor and treat it immediately.
You'd better make an appointment with an ophthalmologist for your grandma for eye exams. In fact, diabetes can cause vision because worse and worse and result in totally vision loss. That because diabetes can damage the lens and retinal of the eyes. So, you shall take her for a treatment to stop it. Just consult an ophthalmologist for a proper treatments according to her healthy condition and eye condition.