How can we help a father who pretends to fast and secretly smokes during the daytime in Ramadan?

Smoking is forbidden and breaks the fast. Doing it in Ramadan is a grave sin. A vision from the Prophet, peace and blessings be upon him, was narrated showing the punishment of those who break their fast before its appointed time, and commented on it: "This is the punishment of one who fasted then deliberately broke their fast before the time of breaking it. So what will be the condition of one who does not fast at all?!"

You should act wisely in advising your father. If you think he will accept direct advice, then speak to him. Otherwise, convey it to him through indirect means, such as playing audio clips or placing written fatwas. If you strongly believe that a greater harm might occur, do not rush, and suffice with praying for his guidance. Be careful not to monitor him or spy on him.