she is in love and her judgement is blinded by it, she thinks everyone is against her relationship with him and it looks like shes chosing his side for now.
you or your parents cant force her to break up at this point, cause she will just choose his side and where does that leave you guys.
its pretty shitty she dropped out school 2 weeks before it finished though, let's hope she comes to her senses sooner rather than later.
the best thing you can do for now is just be there for her as a brother, and try to show her in a friendly, non aggressive towards her bf way, that shes throwing her life away with quitting school/job, and that if she doesnt wanna get treated as a kid, she should be more responsible.
again do this in a nice way, don't try to force it on her, she has to see & believe it herself, cause in the end she's an adult now and makes her own decisions.
also that guy sounds like a fuckin douchebag.