the transparent friend

i used to know a girl who was my best friend. we'd call each other everyday and talk and do all kinds of stuff together. i really, honestly, saw great things out of her because she was smart and pretty and all over great. But then, all of the sudden, over a summer, things changed...
i started noticing a change in her attitude. she questioned innocent little things like weddings in a church or letting a priest do a funeral and called them stupid or gay. She started hanging out with stoners and sluts and people who smoke and drink and shit. She started getting drunk and smoking and all that. And all throughout this, she promised me she was "under control" and that she was "taking care of herself". I knew she wasn't. but i nodded and worried in my mind that something awful was going to happen. Which, it did. One day she called me and told me that she had "done it" with her current boyfriend. I was shocked, but only because she told me so easily! me and my friend's abstinance was our little thing we had together, and she had thrown it away in one night and hated the sex anyway. I felt like a useless friend to her, like i was just a plan B friend who she really didnt want to hang out with because i dont drink and smoke and have sex and throw up all over myself. yeah, to be honest, not a real glorious lifestyle. im aiming to enjoy my life, thanks.
i started hating everything she said from that point on. being her friend became so difficult, but i still tried. i have a mother complex and after getting into this girl's head, i couldnt abandon her. (like she did me) I had to keep trying to get in and help.
she's only gotten worse. She brags about her shitty lifestyle and talks about her vomitting and getting drunk as though they were accomplishments. This girl has no idea what road she has taken, but one think i know, is i miss the nice, sweet nerd version of her. The one that i could easily be friends with and even, at times, look up to. now, she's nothing like that. i wish i could help, but she won't listen...i'll never give up though...im not like that.