What is the ruling on congregational prayer—for both the Imam and the follower—with a person who, after the prayer, turned out to be a polytheistic grave worshiper? And is it obligatory to repeat it?

One must be cautious about declaring a Muslim an infidel unless the conditions are met and the impediments are absent, and this matter is to be referred to the people of knowledge. If it becomes clear that one of the congregants is an infidel, the prayer is valid according to the majority of scholars, because the congregant prays individually behind the Imam, and the prayer of an individual is valid due to an excuse if he does not know the condition of the one standing beside him. However, the Hanbalis consider the prayer of an individual behind the row invalid, and since standing in a row with an infidel does not achieve the intended purpose, the prayer of the congregant is invalid. If there is no one else with the Imam by whom the prayer can be established, then the Imam's prayer is also invalid. The preponderant opinion is that their prayer is valid.