What is the ruling if the Friday prayer imam stands up for a third rakaat, and what may the congregation do without affecting the validity of the prayer?

Summary

When the imam stands for an extra rakaat, followers are prohibited from following him. They may either separate from the imam (mufaraqah) and give salam, or wait in the tasyahhud and give salam after the imam. According to the more sound view, this also applies to the Friday prayer, and the prayer is not absolutely invalidated since the imam may have forgotten or be compensating for a defective pillar. Separating does not invalidate the Friday prayer for those who already obtained one rakaat with the imam, as congregating in the second rakaat is not a condition.

Written in our own words from the original fatwa — check the source before acting on it.