What is the ruling on saying "Ameen" to the Imam's supplication on Friday during the second Rak'ah, and is it true that this act was not reported from the Messenger of Allah, peace and blessings be upon him?

If the Imam performs Qunut in congregational prayer due to a calamity, this is legislated according to the Shafi'is, but not sunnah according to the Hanbalis. It is recommended for the Imam to recite this Qunut aloud, and it is recommended for the congregants to say "Ameen" aloud to his supplication.