What is the ruling on shunning a father who asks his son to buy drugs for him, knowing that buying them would harm the son's reputation, and refusing would lead to him being labeled disobedient?

As long as you are capable of showing filial piety to your father, do not abandon him or cut ties with him. Rather, continue to visit him, fulfill his legitimate requests, and advise him gently. Seek help from someone you trust to advise him, and supplicate for his guidance in private and in public. However, do not buy him narcotic pills if he is not in need of them.