Drops & alt laser failed to keep my 84-year old mother's glaucoma controlled. next step is surgery. how concerned should i be about her vision?

Very: If drops and alt have not controlled the eye pressure , she has probably lost some of her peripheral vision. You need to talk with the glaucoma surgeon. He or she will tell you specifically . What is going on with your mother, and the risks versus the benefits of such surgery.

Concern is normal: Your mother most likely has some changes in her optic nerve appearance and some visual field loss. The reason to suggest surgery is to prevent further damage to the optic nerve(s) and prevent further visual field loss.