What do i do?

My bsf is pulling away from me. I think its cus of her new bf. Her bf has a bad rep at school. He drinks and sneaks out with her to parties. I dont think he's good for her and i told her that. I told her that she's changing and not in a good way, but she tells me i'm wrong and she's the same. But i can see her falling down a bad path and pulling away from me. She used to respond to my texts right away, but now doesnt respond to texts until the next day. She never wants to hang out with me but she's always going out to drink with him. I think shes failing but she used to be a good student. I don't know what to do. She's my bsf since elementary. I want to help her but she keeps saying nothing is wrong.