Is what the mother did by changing the apartment's contract without the father's knowledge, with the aim of depriving the son of it, considered usurpation of another's right?

Leasing is a contract for usufructs, and for its validity, the usufruct and the duration must be known. Leasing "monthly" means setting a monthly rent without a specific duration, and either party can terminate it whenever they wish. Any law that contradicts this is a man-made law, contrary to Islamic law, unjust, and usurpation. A Muslim is not permitted to act upon it or be pleased with it, even if the state authorizes it. Therefore, if the apartment owner wishes to reclaim it, it is their right to do so, and it is not permissible to prevent them based on an unjust law. The family must repent from this injustice.