What should the questioner do after seeing his friend—who frequents the mosque—sitting in a pub with a glass in his hand, not knowing whether it is alcohol or not, and smoking, knowing that his friends drink alcohol?

Your duty towards your friend is to advise him gently and wisely at an appropriate time, and to clarify to him that merely sitting in places where alcohol is consumed is a sin. The sin increases if he drinks himself or sits with those who drink. You should conceal his fault and not tell anyone what you saw, for Islamic law encourages concealing the faults of a Muslim, especially if he is from the people of the mosque, based on the Prophet's (peace be upon him) saying: "Whoever conceals a Muslim's fault, Allah will conceal his fault on the Day of Resurrection," and his saying: "Whoever conceals the fault of his Muslim brother, Allah will conceal his fault on the Day of Resurrection. And whoever exposes the fault of his Muslim brother, Allah will expose his fault until He disgraces him with it in his own home."