Does the prayer become invalid if someone who joined late sits for the Tashahhud in the second rak'ah after the Imam says the taslim?

If you catch two rak'ahs of 'Isha', do not sit during the first of the missed rak'ahs; rather, you only sit at the end of your prayer. However, if you catch one rak'ah of a four-rak'ah prayer or Maghrib, scholars have differed. The majority favor that you perform the tashahhud (definition) during the first of the missed rak'ahs, whether what you caught was the beginning or the end of your prayer. What has passed of your prayer is valid, and you do not need to repeat it. Ibn Qudamah, in Al-Mughni, mentioned the two differing narrations from Ahmad regarding the position of sitting and the first tashahhud for one who catches one rak'ah of Maghrib or a four-rak'ah prayer: The first is that when he stands, he prays two consecutive rak'ahs, begins with takbir, and does not perform tashahhud between them. The second is that he stands, performs one rak'ah, then sits, then performs another rak'ah in Maghrib, or two consecutive rak'ahs in a four-rak'ah prayer. Whatever you do from that, Allah willing, will be permissible.