First if all you should have concrete evidence against that person before accusing him of doing black magic, then another concrete evidence that he is doing it against your father. Before have such concrete evidence against that person, you might fall in a major sin of putting allegation on that person.

You have no right to tell any one about what you think about that person before having the concrete evidence against him.

As far as the black magic which you think is done against your father and his family, the remedy is the same whether it was really done by any one or not. Even if it was not done, it is useful to protect your family by the same recitations which are: Sura Al-Falaq and Sura A-Naas to be recited after every prayer and before going to bed three times, as well as Ayatul Kursi. Sadaqa is also very useful in protection from evil.

Wassalam.