I forget why but it doesn't work, probably because it sends to some email they've never heard of.

It does work. As I said, I had to go through it with a family member a few months ago. It's hardly Apple's problem if a user forgets their password AND email address.

I don't expect Apple to get someone back into an account they've forgotten everything about, but you shouldn't need to do that just to download a free app. Grandma already knows her passcode, that should be enough.