Is it permissible for someone praying Shafa‘ and Witr—three consecutive rak‘ahs with a prior intention—to lead in prayer someone performing the ‘Isha prayer, and what should he do in this situation?

There is no objection to the obligatory prayer behind someone performing a supererogatory prayer, according to the preponderant view of the scholars, such as praying 'Isha behind someone praying Witr. The imam has a choice regarding Witr: either to pray two rak'ahs and make taslim, then stand for the third, or to pray three rak'ahs with one taslim. If the imam makes taslim, the follower stands for the remainder of 'Isha. The difference in the order of prayer does not harm the following; rather, the follower makes Qunut if the imam makes Qunut. Al-Nawawi said: "And if one prayed Dhuhr behind someone praying Maghrib, it is permissible by agreement." Al-Shabramallisi said: "And similar to that is if one praying 'Isha follows one praying Witr in the second half of Ramadan; it is then preferable to follow him in the Qunut."