Talk to Eye Specialist on Glaucoma Vision Loss

My father recently diagnosed with glaucoma. We didn’t know about it earlier, but now his vision is very very poor. He had surgery for glaucoma, and the doctor said the pressure in his eyes is now normal (10). However, his vision has not improved, and we were told it may not get better. He has had diabetes for 15 years and takes insulin. I wanted to ask: Is there any way to improve his vision, even a little?