What is the ruling regarding the prayer of the congregants who completed their prayer after the Imam exited to admonish a worshiper whose phone disturbed him during prayer, and should they have exited the prayer following the Imam?

The Imam erred by cutting off the prayer without an excuse. As for the validity of the congregants' prayer after the Imam cut it off, there is a difference of opinion among the scholars. The majority hold that it is invalid, because any prayer that becomes invalid for the Imam also becomes invalid for the congregants, whether the invalidation of the Imam's prayer was due to an excuse or not. The Shafi'is, however, believe the congregants' prayer is valid even if the Imam cut off the prayer without an excuse.