What is the ruling regarding the mother of the husband asking her son to divorce his wife merely due to a disparity in lineage, despite knowing of the wife's good character and the absence of marital problems?

The superiority of one spouse's lineage over the other has no bearing on their marriage. If a mother commands her son to separate from his wife merely because her lineage is inferior to his, he is not obligated to obey her. He should not divorce his wife as long as their relationship is sound and the wife is righteous. Imam Ahmad explicitly stated that a man should not divorce his wife at his mother's command. Sheikh Taqi al-Din said, "It is not permissible for him to divorce her. Rather, he must be dutiful to his mother, and divorcing his wife is not a form of dutifulness to her." The son must be dutiful to his mother and obey her in what is right and proper.