P.S. Pray for their safety. Witchdoctors have an unusually large amount of power and will not be happy that their influence is waning.
Hopefully the witch doctor will eventually convert. There have been many such cases and they become even stronger leaders in Christ.
I'll grant that they have influence over some people, but that's simply a function of their victim's credulity. Their "magical" powers have no more basis in reality than those of Harry Potter or Gandalf.