I was in a relationship with someone who has an addictive personality, they are currently in rehab. I spoke to an addiction counsellor and she said: "we have to make him see life is better without using". How am I supposed to do that when I think the way I do about life. I am just existing. There's nothing I really want in life.
I don't know what your friend is into, but imagine yourself in his situation—as I infer you are not—dealing with all the negative consequences that inevitably go along with a lifestyle revolving around substance abuse. Dysfunction when you're high, cravings when you're not, being broke a lot of the time, always wondering where and when you'll get your next dose, and so on are only part of the picture. Imagine some of the risks involved, like physical injury from a car wreck, poisoning from a bad batch of drugs, overdose, and—assuming that your friend is hooked on something illegal—having to meet with shady characters, never knowing when one might pull out a knife or a pair of handcuffs. That's an awful lot of down for just a little bit of up. Now take another look at your own life without all of that (again, assuming). It may not be great, but it could be a lot worse, don't you think?
I'm just speaking from my own experience here—your mileage may vary—but I hope it helps.