All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Engagement is only a promise of marriage, not a marriage contract. So, if a guardian of a woman believes that the interest of a woman is to cancel the engagement, he can do so without any reprehensibility. This is the opinion of known Imams; the scholars of the Maaliki, Shaafi’i and Hanbali Schools of jurisprudence are also among them.
However, it is recommended not to cancel the engagement without any reason since it is a kind of not fulfilling one's promise.
Therefore, we say if the fiancé is affected by the mentioned problem and the woman is no longer interested in him, then the guardian of that woman can give up this engagement and can marry her off to another man. In fact, marriage is a contract for life, so any harm may continue till the end of her life. Thus, it is the absolute right of a woman to take all precautions for herself.
We advise our brother to adopt the legal ways for the treatment of magic. Know that Allah does not send down any disease except that He also sends down a remedy for it.
For more benefit on how to get rid of magic, please refer to Fataawa 81667 and 82488.
Allah knows best.