Diabetics are more prone to develop glaucoma , which needs to be checked by an ophthalmologist. Headache with redness indicates this to be the most likely cause. There can be other reasons too . But I would suggest you go to an eye specialist soon as if this has  affected other eye also then it would cause irreversible damage to her other eye vision too .

Hi ... There can be so many things which can cause this..but not able to answer on this platform without checking. Check BP, sugars and take her to eye doctor.

A detailed eye evaluation is required for your grandmother.

Her age,general health,diabetic control n duration ,any history of glaucoma,reason for visual loss,eye pics will help.she requires a eye checkup & fundus evaluation by eye specialist.

Get her eye examination done. She could be having glaucoma which is why the redness is likely to keep coming again and again

Good evening. Her Right eye in which she can't see might have glaucoma increased eye pressure due to diabetic complication in Retina. Please visit Retina Specialist for checkup.

Better take her to eye clinic nearby to check out eye pressure and retina