One of the long-term micro-vascular complications of diabetes is diabetic retinopathy.
It can be prevented (or delayed) by controlling the glucose level in its optimal range.
How high is your mother's blood glucose level and how is your mother trying to regulate her diabetes - with insulin or anti-diabetic pills?
Has she ever had any treatment for diabetic retinopathy?
Headache is a too general symptom and can be due to many conditions.
Does your mother have any other disorders beside diabetes?