Was the worshipper (ma'mum) required to follow the imam in the extra rak'ah, and what should he have done? And is he obligated to repeat the prayer?

The congregants err in following the Imam for a fifth rak'ah. For whoever intentionally and knowingly performs a fifth rak'ah, their prayer is invalid. As for one who is ignorant or misinterprets, their prayer is valid. As for the latecomer (masbuq) who did not follow the Imam in the extra rak'ah, their prayer is valid and they do not need to repeat it.