Mental Mystery. Worried. please help.

There is this friend of mine, who recently returned from the US and is disturbed on a serious level. He does not talk to anybody. Given the fact that he is an introvert, he used to have minimum conversation with people before. But now, he has quit talking to people and just doesn't want to have anybody with him. I have also sometimes found him talk to himself. Or this "imaginary" self.. He is not ready to share anything; just keeps smiling if he is asked a question. It's getting really hard to break him.. He has been smoking a lot and drinking too. The only reason he wants to hang out with us is cuz he thinks he can stay around us and drink or smoke. Since his parents started questioning about his recent habits/ drug abuse, he stopped caring and he has started smoking and drinking at home in front of his parents itself. We as friends are not able to understand what is going on with him and are worried. Please help.