Should the person who pays money to perform black magic be informed that it has succeeded in harming someone, but that there is no share for him in the Hereafter, and that he should fear Allah, repent, and desist from his actions, or should he be left alone and advising him be stopped?

It is not permissible to rely on dreams to prove accusations, because some of them are visions from Allah and some are dreams from Satan. If there is no clear evidence to prove it, then it is not permissible to level accusations. One can offer advice and remind of Allah without leveling accusations. The most important thing is to strive to understand the causes of the symptoms that the questioner and his family complain of and to treat them.