What is the ruling on a mother going to someone who claims to provide treatment and informs her that she is bewitched, even though medical tests have proven she has a hydatid cyst in her brain?

The therapist erred by saying, "Get out," and you were wrong to leave your mother alone with him, as it is impermissible for him to be in seclusion with her. A Muslim must verify therapists, checking if they treat with the Qur'an and Sunnah. It is not permissible to go to sorcerers or soothsayers, due to the Prophet (peace be upon him) saying: "Whoever goes to a soothsayer or diviner and believes what he says has disbelieved in what was revealed to Muhammad." And his saying: "Whoever goes to a diviner and asks him about something, and believes him, his prayer will not be accepted for forty days." Since you did not approve of your mother going, we hope Allah forgives you. The organic illness (a cerebral cyst) might be the cause of the headache, not magic. You can consult doctors again.