Worried in Washington
This is one matter that you can't take the time to play around with, using trial and error. The good news is that your daughter's friend has issued a cry for help and has not yet done the deed. There are several tracks of parallel action to be taken immediately.
First of all, contact a mental health professional who you trust and consult with him. Secondly, tell your daughter that she should remain in close contact with her friend and continue to be supportive. Thirdly, contact the school psychologist, guidance counselor or whoever you trust most within the school to handle the situation properly. In one case like this that I know of, the school psychologist succeeded in making a deal with the girl that if she came to him for therapy, he would hold off on telling her parents.
And finally, perhaps there is something terribly troubling going on in the girl's family and you, as the mother of her friend, can invite her to spend time in your home, if that is an emotional commitment that you are willing to make.