Is it permissible for my mother to claim her debt from my deceased grandmother's estate without the knowledge of the heirs, including my grandfather, knowing that the loaned money was from my father's account?

A mother's disposition of her husband's wealth is not permissible if it is without his permission, due to the Prophet's (PBUH) saying: "The wealth of a Muslim person is not lawful [to take] except with his willing consent." And debt is the second thing to be disbursed from a deceased person's estate, after the expenses of preparing his body.