How should a Muslim son deal with his non-Muslim parents who discuss matters forbidden in Islam with him?

You are obligated to be dutiful and good to your parents, and one of the greatest forms of this is inviting them to Islam. You have done well by inviting them, so continue and do not despair, and supplicate frequently for their guidance. In your دعوتك (invitation/call) to them, focus on faith (إيمان) because it is the foundation of religion, and if it settles in the heart, obedience becomes easy. As for the subsidiary issues, seize the opportunity when your mother mentions them to explain to her the virtues of the Islamic religion by clarifying its prohibition of a woman going out perfumed or taking a foreign man as a friend, especially if she is still married, as mere separation is not considered divorce.