What should be done about the husband's father who resorts to sorcery to separate the spouses?

It is not permissible to accuse anyone of practicing magic without proof. If you are certain that your husband's father is doing this, then it is a grave transgression, as magic is one of the greatest sins, and striving to separate spouses is an act of disobedience. This man must be advised to repent, and whoever knows of the presence of sorcerers should inform the authorities. The cure for magic is easy to attain by relying on Allah, maintaining remembrance of Allah (dhikr), lawful incantations (ruqya), and supplication (dua). There is no harm in resorting to those who treat magic through permissible means.