What should the congregant do if the Imam stands for a fifth rak'ah out of forgetfulness and does not return after the congregants' tasbeeh?

If the Imam stands for a fifth rak'ah out of forgetfulness, the congregants must alert him. If he does not return, and believes he is correct, it is not permissible for a congregant who knows it is the fifth rak'ah to follow him and stand with him. This is because by doing so, he would be knowingly and intentionally adding a rak'ah to the prayer, which invalidates the prayer. Instead, the congregant should sit, recite the tashahhud, and then make the taslim, or wait for the Imam and make the taslim with him. However, if they stand with him out of ignorance, their prayer is not invalidated.