All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
We advise you to try to convince your sister’s husband to forsake this magician and to inform him of the danger of such companionship in this life and the Hereafter.
Try to give him gifts such as books and cassettes that talk about this issue, and try to send some knowledgeable people, whom he respects and obeys, to advise him.
Also, try to persuade him to go to someone to make Sharee`ah Ruqyah (treatment) for him. If this is of no avail and his wife cannot bear living with him any longer, then she can ask for a divorce. If he refuses, the case can be filed in court, which will grant her divorce from him.
Allaah Knows best.