Number one is don't make them feel like you judge them or reject them for it. Continue to be a friend to them and be supportive. You can ask her if she would like to talk to you about it or about what is going on in her life that makes her feel that way. Try to be nonjudgemental - if she feels like you think less of her for it, she will probably not be able to let you help her. She does need to get professional help, and encouraging her to talk to a therapist or counselor is pretty important. If she is not in immediate danger, don't go running to her parents and tell them - that should be her choice, but if she won't talk to anyone and you feel she is in danger, then that might be what you need to do. It is a fine line.
Thank-you for caring about your freind enough to look for ways to help her.

You could also refer her to this message board.
<font color=orange>"If a light beckons to you, follow it. If it leads you into the quagmire, you'll probably find your way out of it again; but if you don't follow it, you'll be plagued for the rest of your life by the thought that perhaps it was your star." Friedrich Hebbet</font color=orange>
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“We should always pray for help, but we should always listen for inspiration and impression to proceed in ways different from those we may have thought of.”
– John H. Groberg