They aren't ruining their own lives; smoking certainly doesn't HELP you but it doesn't ALWAYS hurt you. And it won't ruin the lives of others around them, besides secondhand smoke, which you can avoid and still be their friend. I don't support but I tolerate it and keep the friends that do smoke, because they're my friends. But whatever, if you want to lose a friend because they made a decision for themselves, albeit a bad one, whatever, that's your call.