What is the ruling on what my mother did by placing a paper for my father on apple juice, with something like: "A prayer that incites to prayer" written on it, so that he would drink it and pray?

Magic is a reality and has an effect, including separating spouses. However, it is not permissible to accuse anyone of magic without evidence. A Muslim should be presumed innocent. If what was written is a ruqyah (Islamic incantation) and the bewitched person drinks it for treatment, it is permissible. The bewitched person has known signs, some of which have been previously explained along with the method of treatment.