Is it permissible for a mother to go to a jurist to write something for her son so that he may separate from a foreign woman with whom he is living in sin, knowing that her intention is to save her son so that he may marry a Muslim woman, and does this fall under the category of sorcery?

If the brother is married to a woman of the People of the Book with a legitimate Islamic marriage, there is no sin upon him, even if it is disliked. However, if he is living with her without a legitimate marriage, he is committing adultery and must repent. He should be advised and guided to the right path. It is not permissible to use magic or divination to "save" him, as this is among the gravest sins and incurs the wrath of Allah.