Is a wife's prayer for death upon herself, should her husband smoke, considered an answered prayer, and is there an expiation that nullifies it?

Your wife's praying against herself is impermissible. Rather, it would have been better for her to pray for your guidance and for you to quit smoking, because a Muslim's supplication for his brother in secret is answered. As for the answering of her prayer against herself, its knowledge is with Allah. It might coincide with a time of answering, and thus be answered. For this reason, one is forbidden from praying against oneself, one's children, servants, or wealth. And you must strive to quit smoking, as it is forbidden due to its foulness and harm.