Why doesn’t he know her birthday? The answer to that question might be revealing. I never knew a grandparent other than my maternal grandfather. My other grandfather and both grandmothers died before my birth but I can tell you their birthdays, I can’t recite them from memory but I have the information on hand.
One of my interests is genealogy. I have that kind of info on ancestors back to the 1400/1500’s. It is not hard to find. You just have to look. I have even found info by just typing in the name of the person. I do have one family which I think must have come from Mars because we can not find any info earlier than the early 1800’s. Some in the family changed the spelling back then which does not help matters.