Do the followers return to the tashahhud if the imam is alerted after he has stood up?

The Shaykh : Allah knows best, because she is not addressed regarding attending the mosque; rather, it has been said to her that the house is better for her. He says: If the imam stands up for the third rak'ah, then sits for the first tashahhud, and then after he has fully stood up, he is alerted to sit, does the followers have to return with him to sit? Or does the one who knows the ruling not return? Or is it not permissible to alert him if he has fully stood up? Because some imams are ignorant, so he descends at that moment, is startled, and descends without thinking. Yes. If he returns, they do not return with him. Once they have stood up and fully stood, they do not return with him. But I say: do not alert him if he has fully stood up at all.