The choice of the latecomer to join the imam or not, if the imam forgot a rak'ah and then completed it

The questioner himself: Following up on the question, O Shaykh: Some of the latecomers stood up to complete what they missed, and that was with the first congregation. Are they required to follow the imam when he stands up to perform the missing rak'ah, or may they continue in their prayer? Or what is the ruling on the prayer of the latecomers who did not follow the imam?The Shaykh: This is a second question; we will not answer it unless the brothers permit it. Do you permit it?Those present: We permit it, O Shaykh! The Shaykh: He means to say: This man who forgot a rak'ah and said the taslim after three rak'ahs, the latecomers who missed part of the prayer stood up to make up what they missed. So, if the imam remembers and stands up, are they required to return with him, or do they continue making up their prayer?
They have the choice. If they wish, they return with the imam, and if they wish, they continue in their prayer.