Praise be to Allah.
The answer is: If your old wife admitted doing this or it is definitely proven that she did it, then she has committed a great evil, indeed it is kufr (disbelief) and misguidance, because what she did was forbidden sihr (witchcraft or magic), and the one who does that is a kafir (non-Muslim), as Allah says (interpretation of the meaning):
“They followed what the shayateen (devils) gave out (falsely of the magic) in the lifetime of Sulayman. Sulayman did not disbelieve, but the shayateen (devils) disbelieved, teaching men magic and such things that came down at Babylon to the two angels, Harut and Marut, but neither of these two (angels) taught anyone (such things) till they had said, ‘We are for trial, so disbelieve not (by learning this magic from us).’ And from these (angels) people learn that by which they cause separation between man and his wife, but they could not thus harm anyone except by Allah’s leave. And they learn that which harms them and profits them not. And indeed they knew that the buyers of it (magic) would have no share in the Hereafter. And how bad indeed was that for which they sold their own selves, if they but knew.” [2:102]
This ayah (verse) indicates that witchcraft is kufr (disbelief) and that the one who practices it is a kafir. Those who practice witchcraft learn that which harms them and does not benefit them. One of their aims is to separate a man and wife, and they will have no chance before Allah on the Day of Resurrection – i.e., they will have no hope of salvation. According to a saheeh hadeeth (authentic narration) narrated from the Messenger of Allah (peace and blessings of Allah be upon him), he said, “Avoid the seven things which will lead to doom.” He was asked, “What are they, O Messenger of Allah?” He said: “Associating anything with Allah; witchcraft; killing a soul whom Allah has forbidden us to kill unlawfully; consuming riba (usury, interest); consuming the orphan’s wealth; fleeing the battlefield; and slandering chaste but innocent believing women.”
With regard to the Shaykh who gave you the remedy, it appears that he is a practitioner of witchcraft like the woman, because no one knows about actions of witchcraft except others who do the same. He is also one of the fortune-tellers and soothsayers who are known for their claims to know many matters of the unseen. The Muslim must beware of them and not believe their claims to know the unseen, because the Prophet (peace and blessings of Allah be upon him) said: “Whoever comes to a fortune-teller and asks him about anything, his prayers will not be accepted for forty days.” [Muslim]. And the Prophet (peace and blessings of Allah be upon him) said: “Whoever comes to a fortune-teller or soothsayer and believes what he says has disbelieved in that which was revealed to Muhammad (peace and blessings of Allah be upon him).” So you have to repent and regret what you have done, and tell the head of the organization and the head of the court about this Shaykh and your old wife, so that the court and organization will take action to deter them.
If something like this happens to you again then ask the scholars of sharee’ah (Islamic law) to tell you of the remedy prescribed in sharee’ah. If what happened to you has now gone, then praise be to Allah, otherwise let us know so that we can tell you of the remedy prescribed in sharee’ah. May Allah bless you and us with understanding of Islam and steadfastness in our religion, and may He keep us safe from that which goes against it, for He is the Most Generous, Most Kind.
And peace be upon you and the mercy of Allah and His blessings.