I think where you went wrong is, you said, "yes."
Your answer should have been, "No. You're a slacker and a bad influence. You should be apologizing to her, not me, for dragging her down."
Once you take a moral stand (that her new friends are bad for her) you should never back down until you win or you are proven wrong.
Ultimately, you are not her mother. She will make bad choices and you can't stop her. You might have to cut your losses and let her go, but don't apologize for being right or doing the right thing.
you are correct, when helping your friends, you can't be their friend, you have to lay it on them straight and tell'em how it is. It's for their own benefit