What is the ruling on the follower continuing with the imam if the imam stands for a fifth rak'ah out of forgetfulness after the followers have alerted him, but he did not respond?

If the Imam stands for a fifth raka'ah by mistake, and is reminded but does not return, then whoever follows him, knowing of that addition, his prayer is invalidated according to the majority of jurists. As for the ignorant person, his prayer is not invalidated. The latecomer (masbuq) and others are equal in this regard, and it is better to wait for the Imam.