I've liked a girl I know for about a year or so now. The problem is she's been dating someone for almost a year. The boy she's dating is younger than her, and in my opinion, way too immature for her. She and I have been close friends since before she started dating this kid, and she's told me that their relationship is starting to show cracks. What should I do?

Nothing. Leave it alone. If she breaks up with him, fine. If she gets the relationship sorted out and is happy with him, you'll have to live with it.

What you must not do is anything which would speed up the end of the relationship. Not only is it nasty, but it will poison any future relationship you have with her.

She's in another relationship - that means she's totally off limits.

You really need to stay out of it.

Not as easy situation to be in. I feel for you, but check this sticky out for insights: https://www.askmehelpdesk.com/dating/guidlines-what-do-do-if-person-like-already-relationship-463250.html

Went through this recently, you have to back off for now.

On the plus side, while yes, she is off limits when it comes to telling her how you feel or making any move, you can always talk to her. Talking to her can involve flirting and getting closer, but you have to be careful when you do this. There are several reasons to be careful: you can get too close/personally invested, you can get stuck in the friend zone, or you could miss the real girl you are supposed to be with. So, the best way to go about this is to be hot and cold with her, talk to her a bunch one day and not talk the next. Go and do your own thing, she will still want to talk to you if she is interested and will probably respect you a ton more for doing your own thing. You cannot be the reason they break up, nor would you want to be. Just go and live your own life, you may meet someone who is available that you like more, and if you are living your own life and having a good time, this girl you are talking about may want to experience it with you too! If you are sitting at home waiting for her... who would want to be a part of that?