I snitched on my best friend for smoking.
Why did you snitch on your friend for smoking?
I did it because I want her to quit smoking.
It's good that you're concerned about your friend but you betrayed her by telling on her. You have to find more supportive ways of convincing her to stop smoking.
I'm making her a better person. She can't make it without her friends.
I know you meant well, but you'd be better off talking to her about this rather than "turning her in." She's unlikely to quit smoking unless she wants to herself.