What is the ruling on dealing with someone who claims to know the unseen, using papers and amulets for healing, and is it permissible to share meals with him, greet him, and pray behind him, given that he sometimes leads people in prayer and refuses advice?

It is not permissible to pray behind soothsayers who use devils and draw near to them through sacrifice and invoking their names, and obeying them in disobeying Allah by consuming unlawful things and impurities, because this is an act of disbelief and polytheism. They must be asked to repent; if they repent, well and good, otherwise they should be executed.