Is it permissible for my father to buy houses in his name using my grandmother's money, knowing that she is currently not compos mentis, based on her previous authorization to him and her saying, "My right is for you, my children," and with the approval of her heirs?

If the power of attorney was obtained after the grandmother lost her mental faculties, it is void because she has become interdicted. If the power of attorney was granted when she was in good health and of sound mind, it becomes void upon the deterioration of her mental faculties. The father's management of his mother's wealth must be solely for her benefit. Nothing should be taken from her wealth except what she needs. It is impermissible to undertake actions that harm her interests, such as registering a portion of her wealth in the father's name, as this is a prohibited falsehood. The grandmother's wealth remains her property as long as she is alive.