If someone knows that their father watches forbidden things on his mobile phone, what should he do?

If a son becomes aware of an evil deed committed by his father, he must advise him gently, kindly, humbly, and with good speech, while supplicating for his guidance. If that is difficult, he should seek the help of someone who can influence his father with general advice, without explicitly mentioning the sin, in order to conceal it. It is not permissible to be harsh with the father or address him in an inappropriate manner, as Imam Ahmad, Al-Ghazali, and Sheikh Abdul Aziz Al-Rajhi have stated the obligation of gentleness and kindness in advising parents, and avoiding intimidation, threats, insults, or harshness.