Is it permissible to pray for someone who used to frequent sorcerers, believe them, and deal with them, then became mentally ill, stopped praying, and committed suicide, knowing that he used to pray and perform good deeds when he recovered?

It is not permissible to visit sorcerers and believe them. If a man abandoned prayer and committed suicide due to a mental illness, and he prays when he recovers, then the funeral prayer must be performed over him. However, if he intentionally committed suicide, the correct opinion among scholars is that the funeral prayer should be performed over him like other Muslims. The ruler may choose not to pray over him as a deterrent against his action, but the funeral prayer should not be abandoned completely.