Does the one who joins the prayer late follow the imam if he stands up after the imam has finished to complete a rak'ah he forgot?

Question:
May Allah reward you. The questioner says: A man entered with an imam who was praying Maghrib, and he had missed a rak'ah. When the imam completed the prayer and said the taslim, the imam then remembered that he had forgotten a rak'ah, so he stood up to pray. The ma'mum had completed the prayer. What is required of him? Does he follow the imam or say the taslim?

Answer:
If the imam stands up to complete a rak'ah he forgot, and the ma'mum has already finished, and he stands up to complete what is required of him, he has a choice. They say he has a choice: if he wishes, he joins the imam, and if he wishes, he remains alone. Yes, he has a choice between the two. Yes.