Should a congregant who joined the Imam in the second raka'ah pray as if no raka'ah was missed, then recite the tashahhud with the Imam and thereafter make up what was missed, or should he remain silent during the tashahhud and complete his prayer?

The latecomer is commanded to join the Imam in whatever state he finds him. If he catches the Imam in the tashahhud, he should sit with him and recite the entire tashahhud. He should not make the taslim with the Imam, but rather stand up after the Imam's taslim and complete what he missed. This is what the Permanent Committee for Scholarly Research and Ifta and other scholars have ruled, based on the Prophet Muhammad's, peace and blessings be upon him, sayings: "When one of you comes to the prayer and the Imam is in a certain state, he should do as the Imam does," and "Whatever you catch, pray, and whatever you miss, complete."