How can one deal with a father who has become harsh and resentful towards his children and wife, and who associates with those younger than himself, and should he be confronted, or is it sufficient to pray for him, knowing that he suffers from high blood pressure and diabetes?

The rights of parents are among the most obligatory rights after the right of Allah. It is not permissible to harm one's father by blaming him for his interest in his appearance or for accompanying those younger than him, especially if doing so would affect his health. Nor is it permissible to think ill of him merely for such reasons. If you confirm that he is committing violations of Islamic law, then the obligation is to advise him with gentleness, kindness, and good admonition, avoiding making him angry, while supplicating for his well-being.