The ruling on praying 'Isha' behind one who is praying tarawih

Question:
A man entered with the imam and did not pray 'Isha'. This man, while the imam was in tarawih, prayed with the imam?

Answer:
There is no harm. If he intends 'Isha' behind one who is praying tarawih, it suffices, just as Mu'adh prayed with his companions.
Q: But the question, O Shaykh, is that when the imam said the first taslim of tarawih, this man stood up to make up his prayer. Then when the imam began the second, he joined him again?
The Shaykh: Did he make up the rest of his prayer alone, or with the imam?
Q: No, when he stood up to make up his prayer, the imam said takbir for tarawih, so this man joined him to make up with him?
The Shaykh: You mean he intended it behind him?
Q: Yes.
The Shaykh: It suffices, there is no harm, no blame. Praise be to Allah, there is no harm. His prayer is behind two taslims.
Q: Is this better, or is it said that he should suffice...?
The Shaykh: No, it is better that he completes it. If his imam says taslim, he completes it alone.
Q: Is it not permissible for him to say taslim with the imam and then stand up?
The Shaykh: He does not say taslim... Mu'adh used to pray 'Isha' with the Prophet (peace and blessings be upon him), then he would pray a voluntary prayer with his companions while they were still in their obligatory prayer.