Well, even if they did tell him, and he remembered who she was, it wouldn't be long before he forgot they told him. Don't get me wrong, I'm not making fun of him; just stating the reality of the disease. My mom was diagnosed with Alzheimers back in 1998; she's as the point now that she doesn't remember that my Dad died 21 years ago, and she sometimes gets to the point where she gets agitated because her Mama and Daddy will be worried because they don't know where she is. Her Mom died in 1992 and her Dad died in 1950. We always assure her that yes, they know where she is!
She still remembers her kids, but gets kind of fuzzy on the grandkids, especially our kids who she only sees once a year! But when we saw her this summer, we got a great picture of her with my family and I'll treasure it.