What is the ruling on going to a woman who claims to heal through charlatanism and sorcery, and wearing a suspicious thread from her? Is there sin upon the one who does that, and is repentance required of him?

It is not permissible to go to sorcerers, or to ask them, or to believe them, due to the hadith: "Whoever comes to a diviner, or a sorcerer, or a soothsayer and asks him, and believes what he says, has disbelieved in what was revealed to Muhammad." And merely going to them is forbidden, due to the hadith: "Whoever comes to a diviner and asks him about something, his prayer will not be accepted for forty days." If you went to them under pressure or thought it was permissible, we hope there will be no sin upon you, but it is safer to repent to Allah, for repentance erases sins.