We prayed the midday prayer in three rak'ahs, then the imam pointed out the omission, although some of the worshippers had already left. What is the ruling on their prayer?

Question:
He says: Honorable Shaykh, may Allah grant you success. He says: We prayed the midday prayer today in three rak'ahs, then the imam pointed out the omission after the end of the prayer, so he turned around and came back to complete the rak'ah. His question is: There was a man who left immediately after the prayer, and he was next to me, and I know him. What is the ruling on his prayer, and is it obligatory for me to inform him of this? Yes, it is obligatory for you to inform him.

Answer:
He must repeat the prayer because the interval between completing the prayer with the rak'ah was prolonged. Since the interval was prolonged, he repeats the prayer from the beginning. Yes. As for if he left and did not know, he is excused. But as long as you see him, inform him. It is obligatory for you to convey this to him. Yes.