Remember that being someone’s friend doesn’t mean always agreeing with them.

And sometimes disagreeing with them means that sometimes, while you’re respectful of their making their own choices, you’re not going to enable or participate in those choices. If a friend or loved one persists at doing something you don’t agree with—that seems to only make him feel worse—in your presence, then I suggest responding in one of two ways: either withdraw your presence, or withdraw your involvement in what he's doing. In other words: "I'm not going to control or lecture you. But I'm not part of this, either."