What is the ruling regarding the Friday prayer for a group that prayed outside the mosque, separated from the Imam, due to a power outage and their inability to hear or see the Imam, while the Imam was praying with another congregation inside the mosque?

If the Imam's voice cuts off and the worshippers outside the mosque cannot see him or the congregants, and they put forward one of their own to lead them in Jumu'ah (Friday prayer), their prayer is valid. In this case, the Imam is considered as being unable to complete the prayer, and there is no harm in putting someone else forward, just as happened with 'Umar ibn al-Khattab when he was stabbed and put forward 'Abd al-Rahman ibn 'Awf to lead the people in prayer.