Try to talk to her calmly and explain why you're worried - being a sibling hopefully means you have a better chance than most people at being able to find out why she's angry and why she's getting herself into trouble. While it's possible that she might just be acting rebellious out of malice, don't discount the possibility that she's actually struggling with issues that she's kept hidden from everyone around her and you don't know about - possibly things she thinks you won't understand or that she's embarassed about. The more you can get her to open up to you the more likely you are to be able to help her.
Whatever you do, avoid giving her lectures or passing judgement because she'll probably just shut you out - even a problem which seems silly/petty to you might be a big deal to her. She probably perceives the things she's doing as being 'grown up', and his not being a good guy might be part of her attraction to him, so anything you do which makes her feel like you're treating her as a child or that she perceives as not respecting her choices could backfire. The best way to help her is to be that person who she can trust enough to open up to knowing that you'll be supportive regardless of any bad choices or mistakes.