What is the ruling on the prayer of a congregant who completed four rak'ahs with the Imam, then made up a missed rak'ah and prostrated for forgetfulness, while the Imam added a rak'ah to the prayer, did not prostrate for forgetfulness, and then departed?

The Imam's prayer is valid if he errs and adds an extra Rak'ah and forgets the prostration. However, for anyone who knowingly follows him in the extra Rak'ah, their prayer is invalid. If one follows him unknowingly due to error, their prayer is valid, provided they perform a Rak'ah to compensate for the one they missed. This is the correct view in the [Maliki] Madhhab (definition). So, if you followed him unknowingly due to error, your prayer is valid.