What is the ruling on the prayer of a congregant who did not follow his imam in an extra prostration in the second rak'ah of the Fajr prayer, and then performed the prostration of forgetfulness with him after the taslim?

The congregant was correct in not following the Imam in the certain addition, for had he followed him, his prayer would have been invalidated. Similarly, he was correct in performing the prostrations of forgetfulness with the Imam.