The vision standards for driving fitness are set at the state level and vary state by state. I'd recommend checking to see what her state's regulations are. Her vision impairment
may clearly place her in the "unsafe and not legal to drive" category and that will make the discussion about whether she can drive or not moot.
But her emotional reaction is normal and understandable. We don't have great options when you stop driving and I'm sure she does feel like a prisoner (or a bird with the wings clipped as another person described it). Acknowledge and honor her frustration with the situation. But then remind her that she has had to overcome other losses and setbacks over her life. She has the coping skills and support to make it. This is another challenge that will require grace to get over. Ask her to think about ways that would make her feel less dependent on others for rides (e.g., maybe a standing 4 hour block (Tuesdays 10-2) when someone will drive her places).
It can be hard to hear the frustration, pain, and maybe even fear that she feels. But talking openly and frankly will be helpful for her and your family. None of us looks forward to the day when we are dependent -- we need kindness, love, reassurance and support to navigate this new terrain.