Therefore, you must keep reiterating the explanation to her, warn her with the fear of Allah, and make it clear that her present practice does not absolve her from responsibility. That is all that lies within your power. If she corrects herself, then all praise is due to Allah; if not, you have nonetheless fulfilled your duty. Ask Allah to guide her.
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The Follower Has a Choice
Question: When the imam pronounces the taslīm even though the prayer is deficient, a congregant who arrived late stands to make up what he missed. The imam then realizes the deficiency and rises to complete the prayer, while the follower has already prayed one rakʿah and still owes another. Should the follower rejoin the imam, or should he continue on his own? And must he perform sujūd al-sahw (the prostration of forgetfulness) or not?
Answer: If the imam has said the taslīm and the latecomer has stood to perform what he missed, then the imam recalls the deficiency and stands to complete it, the follower at that point is free to choose: he may either proceed independently and finish his own prayer, or he may re-enter behind the imam and follow him in what remains. He is at liberty to do either, and Allah knows best.

Question: Even if we assume that he has already prayed one of the rakʿahs he missed, may he still rejoin the imam afterward?
Answer: There is no objection to that. Suppose the imam gave the taslīm after two rakʿahs of a four-rakʿah prayer, and the follower had missed three rakʿahs, having caught only one with the imam. The imam then rises, and the latecomer prays one or two of the rakʿahs he missed so that only one remains. When the imam stands to complete the prayer, the follower may join him for the remainder, even if what is left equals or is less than what he still owes. He may rejoin the imam and follow him in completing the prayer.