Is it permissible to follow a latecomer while he is completing his prayer?

Question:
May Allah reward you, if Allah wills. In the 'Isha' prayer, a man who had missed a large portion of the prayer was behind the imam. Then a late group arrived, and they placed this late ma'mum (one praying behind the imam) in front, thinking he was praying alone. The question is: What is the ruling on the prayer of the late ma'mum? And what is the ruling on the prayer of the late group behind him? And how should a person handle this situation if it is not permissible?

Answer:
This is contrary to what is preferred. The preferred course is that, since they are a group praying alone—a group that missed the prayer—they should appoint one of them to lead and pray as a congregation. They do not need the one who is standing to make up what he missed. He should complete his prayer alone, because he is part of the first imam's congregation. He completes his prayer alone, while they establish a congregation from among themselves. This is the preferred course and avoids the disagreement. In any case, if someone does this, we do not say that your prayer is invalid. Their prayer is valid, but it is contrary to what is preferred and contrary to what is best. Yes.