What should a congregant do if the imam leaves the prayer due to a legitimate Shar'i excuse: should he complete the prayer alone, or should he exit from it?

The worshiper must complete his prayer individually and not قطع it. This is the view of the majority of jurists. Asbagh disagreed, stating that it is not permissible to continue the prayer in this situation, and the worshiper must قطع it and restart it.