What is the ruling on obeying the mother in informing the grandfather about the addicted father's behavior, given the father's threat to disown his son if he does so?

You must deal wisely with parents to earn their رضا (good pleasure) in a way that pleases Allah Almighty. Advise the father addicted to alcohol, reminding him of the dangers of his addiction and the verses and hadiths that prohibit alcohol. If direct advice does not work, you can seek help from close relatives or present a recorded admonition. If there is no reconciliation between the parents, the mother's right takes precedence over the father's right. The mother should advise her husband and pray for his guidance. If she cannot endure living with him and is harmed, she has the right to take her case to the Sharia courts to have the harm removed from her.