What is the ruling on watching pornography, and what do you advise someone who watches it, is a smoker, and is difficult to advise?

Continue to advise and guide your relative to abandon looking at lewd images and smoking. Vary your methods and approaches of dawah (inviting to Islam), and do not despair of his guidance, for the hearts of people are between two fingers of the Most Merciful; He turns them however He wills.