Is a father sinful if he severs ties with his daughter and refuses to visit her, and prevents the mother from visiting her, due to his displeasure with her marriage, even though it was conducted according to Islamic law (Shari'a)? And what can be done to convince the father to reconcile?

If the father did not approve his daughter's marriage but reluctantly submitted to the situation under duress, threatened with recourse to man-made law, then the marriage is invalid (batil); due to the Prophet's (peace be upon him) saying: "Any woman who marries without the permission of her guardian, her marriage is invalid, her marriage is invalid, her marriage is invalid."