Provided that your mother never moved, or that her address at the time the divorce was filed has been known to your father, then clearly there is a breach in the process of properly serving her the summons. Your father and his attorney could face sanctions as a result of this. Your mother may also has the right to an independent action in equity to set aside a default judgment as void for defective service of process. Please work with an expert divorce attorney who could help clarify more about your mother’s rights and the proper legal action she can take regarding this matter.