What is the ruling on dealing with a drug-addicted father who refuses treatment, squanders his money, and mistreats his family, and is it permissible to abandon him?

It is not permissible to abandon one's father, no matter his condition. One should deal with him kindly, while attempting to cure him of addiction. If his harm intensifies, the matter should be referred to the competent authorities, and one should persistently supplicate and humbly entreat God for his guidance.