Is it permissible to pray behind an imam and preacher who attends a "waʿdah" where sacrifices are made to idols?

Slaughtering for other than Allah, as an act of worship and submission, is major shirk (polytheism) that expels one from the faith. Whoever attends a slaughter for other than Allah while disliking it or being ignorant that it is shirk, is not a polytheist. However, whoever knows it is shirk and approves of it, is a polytheist, based on the Almighty's saying: (Indeed, you would then be like them). The Imam who approves of shirk, prayer behind him is not valid. As for one who dislikes it or is ignorant of it, he must be advised. If it is possible to pray behind someone else, that is preferable. But if boycotting him leads to missing Friday prayer and congregational prayers, then he should not be boycotted; rather, one must pray behind him, acting upon the practice of the Companions.