What if..

What if there is this thing, that you know that it is killng the one that you love the most, yet the loved one can't live without it, what will you do? He wants you to know as that will make help him in reducing his stress and he said will help him to quit the problem. But it's just you, after knowing it, you will find ways to stop him from doing it, yet you don't want to be the one who gives him another source of stress, so you decided to be supportive to him by letting him take it. But how could you? Supporting him to take something harmful? But if he'll not tell you, he'll take more behind you and he'll have to lie to you, how could you? What I do? I cry alone, smile to him and keep quiet. I don't know a better way.