Is the prayer of one who goes to a person who claims to heal and writes papers with matters that are not from the Qur’an and Sunnah, and invokes names other than Allah, accepted? And what is the ruling on supplicating against this person?

If the person mentioned does not pray and then enters the mosque to pray, this is a contradiction in the description. As for the ruling on a soothsayer or sorcerer, their prayer, and praying behind them, relevant fatwas can be consulted. Going to soothsayers and sorcerers and adopting their practices is a grave evil, and a warning has been issued that the prayer of one who goes to them will not be accepted for forty nights, or that one who believes them disbelieves in what was revealed to Muhammad. The non-acceptance of prayer means the absence of its reward, not its invalidity. As for supplicating against the soothsayer or sorcerer with what they deserve, there is no harm in it. However, those who go to them should be advised, the truth should be explained to them, and they should be prohibited from evil. It is not permissible to supplicate specifically against an individual because of their sin, unless they have wronged and harmed others.