According to Islamic scholars in Shafi fiqh, if a latecomer joins the imam during the final tashahud and sits with the imam, then after the imam finishes and the latecomer stands to complete his prayer, he should not recite Dua Istiftah. However, if the latecomer joins the prayer without sitting with the imam and directly stands for his prayer, then he may recite Dua Istiftah. This is the Islamic ruling mentioned in classical fiqh books and supported by scholarly fatwa.