What to do when husband with Alzheimer's asks if his mother is still alive, but died many years ago, before I knew him.

My friend and her husband had a beloved dog, and after husband went to memory care, she often took the dog to see him. He kept Fido for the day sometimes. Unfortunately, Fido died, and friend didn't know whether to tell her husband or not. So she didn't.

The thing was, her husband figured it out after a while because Fido stopped coming to see him. So one day he said, "Fido has passed, right?"

She very gently said yes, and husband looked sad, but not for long.

Your husband may not ask again, but if he does, you could keep it brief. Such as, "Poor Fido passed in November, and it was very peaceful. Ready for another donut?" Or you could just say you don't know if his mom is alive and then change the subject.

You can either ask him when he last saw her to continue
a conversation of his good memories or you can tell him that you have not seen her in a while

Tell him the truth. Diversion. All the usual things. Some things go forward in an obsessional way, but many times he may move on to something else quickly enough.
Help him with making scrapbooks of years, and his Mom and his life. It won't necessarily help but he may enjoy it. If he DOESN'T--if it adds to his distress--stop at once.

You didn't now it wasn't kind , you where probably shocked and didn't know how to respond. Maybe now ask him more about his childhood memories and his mom, and maybe he will mention what his mom liked to do, then when he does ask you can say something related to what he told you. Like she went to the park, or whatever his mom liked doing

It depends on how he reacted when you told him his mom died.
If he was upset I would not tell him again.
The way you phrased his question it sounds to me like he may have just been confirming what he knew to be true.
If he wants to talk about her you could ask him what she was like. What she did, what meals she liked to cook and all sorts of questions.

If he doesn't remember that his mother is dead, don't tell him. All that will do is upset him and it will be like he's hearing of her death for the first time in his broken mind. That's cruel and totally unnecessary.

The kind thing to do is to just tell him that she's doing fine and that she's looking forward to seeing him soon.