Is it considered disbelief to believe the sorcerer regarding the one who performed the magic?

It is not permissible to accuse anyone of sorcery, nor to go to sorcerers, question them, or believe their claims of knowing the unseen. Your aunt erred by going to the sorcerer if she knew he was a sorcerer. Believing a sorcerer's claim to know the unseen is an act of disbelief (kufr). However, believing him in what he claims to achieve through sorcery does not constitute disbelief for one who believes him, if it is actually happening. Nevertheless, it is not permissible to believe a sorcerer's accusation that a specific person has performed sorcery, as this involves accusing people without evidence and accepting slander. As for going to someone who performs lawful ruqya, it is permissible.