Quaeritur: Is free will a philosophical or theological topic?
Respondeo: It is both. Nothing prevents two sciences from studying the same object, so long as they consider that object from different formalities, aspects, or points of view. This is why in Scholastic thought there is a distinction between the 'material object' and the 'formal object' of a science. The material object is the thing studied, and the formal object is the point of view, perspective, or angle from which it is studied. For instance, Anatomy and Physiology have the same material object: the human body. But they study it from different points of view. The formal object of Anatomy is the structure or parts of the body the formal object of Physiology is the functions of the body.
The same is true with free will. It can be dealt with in both philosophy and theology, but from different points of view. When the philosopher makes free will the material object of his studies, he has to study it from the point of view of human reason and experience; whereas when the theologian makes free will the material object of his studies, he has to study it from the point of view of divine revelation. The same is with other topics: God, creation, the human soul, human happiness, human acts, virtues, the passions, natural law, etc. are all studied under different formal objects in philosophy and in theology.