The ruling for one who joins the prayer late if the imam prostrates for forgetfulness after the closing salam

If the imam prostrates for forgetfulness after the closing salam, how does one who joined the prayer late act?
If the imam prostrates after the closing salam, one who joined the prayer late does not follow him. Rather, when the imam concludes the prayer, the one who joined late stands up and makes up what he missed. As for the prostration, if he witnessed the imam’s forgetfulness, it is obligatory for him to prostrate after the closing salam. If the imam forgot before this one who joined late entered the prayer with him, then he owes no prostration.