Does sin or expiation devolve upon a wife who lived with her husband for a period during which he did not pray, knowing that she was unaware of the impermissibility of that until recently, and then the husband repented and became observant of his prayers?

If your grandfather abandoned prayer out of laziness, not out of denial, then he does not become a kafir by that according to many scholars, including the three Imams. And since his repentance was sincere and he became observant of his prayers, Allah accepts repentance and erases what has passed. Your grandmother should repent to Allah for her negligence in advising him if she knew the ruling on abandoning prayer and fell short in advising him. There is no expiation upon her for that. If she advised him or did not know, then there is nothing upon her.