What is the correct way to complete the Maghrib prayer for someone who joined the Imam in the second rak‘ah, then the Imam, out of forgetfulness, said the tasleem after the first tashahhud, before completing the prayer?

If the Imam, by mistake, says the salutation before completing the prayer, and the latecomer (masbuq) stands up to make up for what he missed, then the Imam realizes his mistake, the latecomer must return to follow the Imam, even if he has already stood up completely. What the latecomer did before returning to follow the Imam is not counted.

Some scholars are of the view that the follower (ma'mum) has the choice between completing his prayer individually or returning to follow the Imam, even if he has already prayed part of what he missed.