Ruling on a latecomer who left the imam, then the imam returned for forgetfulness

Question:
A questioner says: A man arrived and the imam had already prayed one rak'ah of Maghrib, so he prayed one rak'ah with him. When the imam prayed the second rak'ah, he made a mistake and said the taslim after two rak'ahs. This latecomer stood up to make up what he had missed. Then the imam was reminded and stood up to make up what he had missed. What is prescribed for the one who left the imam to do?

Answer:
He should return with the imam and complete the prayer with him, then make up the missed portion after the imam's taslim. He returns with the imam to complete the second rak'ah, then makes up the third rak'ah, unless he had already left, in which case he has fulfilled what is required of him.