So then my mother found out you can just renew your license online, but she can't decide if she should tell my grandmother or not. Ack.She should not tell her. If, heaven forbid, your grandmother has one accident and hurts herself, or someone else, your mother will never be able to forgive herself. Let her get her eyes tested at the registry. At least she'll know if she can't make the minimum standard.
I say this, as someone who is not prejudiced against seniors driving. My grandparents drove, and drove capably, way up in their 80s. My grandfather stopped driving at 89, when his car died, and he couldn't see the sense of buying a new one (because he was frugal, and he doubted he'd live long enough to get his money's worth *g*). He was still in possession of all his faculties, and in remarkable condition for his years. He died a month shy of turning 91. Until about the last month of his life, he was well enough to drive. My grandmother drove until she was about 84, when she was having trouble with her eyes, and lived, other wise well, and with it, until she was 86.