What's happening? Whose fault is it?

Ever since kinder me and my best friend have been inseparable. But over the past two years we have changed, got new friends and got new interests. That’s fine. It’s understandable. We can’t be the same little kids forever. Even though we have grown apart we still made an effort to keep in touch and we caught up regularly. Except things have changed over the past few months. She has changed. More than normal. She’s hanging out with people that are bad news. She’s going out every weekend and getting drunk, I know it doesn’t seem like a big deal but she is only 14. Her new “friends” are not really her friends because I’ve had dealings with them before. And I had a fight with them because they used me. So we are basically enemies so now they have turned my best friend against me. I told her the other day how I felt. She burst into tears and said how she felt bad and she would stop hanging around with them because she values our friendship more than there’s. But the next day things went back to normal. She was hanging around them and bitching about me. I’m soo confused. I don’t’ know what to do. Is there anything else I can do?