friends vaping/smoking in highschool (I need advice)

I didn't want to post this publicly because I didn't want my friend to see it, but I really need advice.

I haven't been able to understand my feelings recently about how my friend started vaping and possibly smoking weed. We're in 10th grade, and I get that this is the age that some teens start to experiment with that kind of thing. I just feel scared and shameful because I think that they started doing that when I upset/stressed them out last year and we stopped being friends.

I know that it's not any of my business, but since we were really close I feel sad that they started vaping. Anytime I think about them doing that I get really anxious and it's pissing me off because I know that there's nothing I can do about their actions. I think the main thing I've been worrying about is when they talk about selling/trying vapes with someone in my homeroom in front of me. We sit together at a small table, and they talk about flavors of things and the selling of things and they use very suggestive wording so I know it's about vapes.

Addiction is very serious, and I understand that completely. It's not like they are going to stop hanging out with the people who vape and stop going out of class to vape because they want to quit vaping, that's extremely difficult. And if you try to quit vaping, you won't be as close to the other people who vape anymore and you'll have no one else to replace that feeling so you get depressed. It feels good to have someone who does addictive things with you rather than doing it alone.

But this might ruin their life. I remember in 8th grade they said they would never do anything like that, especially because their dad is addicted to cigarettes and it had been hard for them. Makes it worse that they have addictive genetics. But of course everyone changes their mind from when they were younger. 

I don't know how to stop thinking about this. I don't know what to do, and I don't want to talk to them about it right now because I don't want to start anything. I was already VERY stressful in 9th grade (I MOST likely had something at least similar to OCD, it was really bad). I had a lot of stupid serious talks with them and I know that those had to have been stressful. But I've changed, so I'm trying to maintain that. 

If anyone has any advice for me to stop thinking about this or how I can tell them that I feel uncomfortable when they talk about selling vapes in front of me, please give it.