Let's just say, a friend makes a bad decision for there health, like smoking. It would be important to persevere over this situation, mainly, because, even know it's a difficult situation (And hopefully you wouldn't/didn't have to go through this.), you have to take action for their, and your, benefit. To do this I would tell a responsible adult about the situation. And if a adult says it's ok I would watch out for my "friend". I would also keep myself occupied with other things so I wouldn't dwell on the situation. (This is a situation mainly set for school aged people.)
I hope this does help delve deeper into this subject, because it is a very important one to learn and a good one to improve on. I myself hope to improve on persevering and will try my best to do so.