I saw in the cemetery a man wearing jeans and Tshirt smoking.

Answer

Once when Hadhrat Hasan Basri (Rahmatullah alayh) saw a man eating bread in a Qabrustaan, he commented that the man was a 'munaafiq'. The chap with the jeans smoking at the graveside cannot be a Muslim although he may have been born in a Muslim home. His appallingly evil conduct testifies to his kufr. It is haraam to smoke even outside the cemetery or anywhere else.