I feel very sorry for you my dear.
You are a very good friend to do what you are doing.
I hope you know that :)
You have to remember that although your friend doesn't want to tell an adult about her situation it cannot be all placed on your shoulders.
I cannot even imagine how worried and stressed you must be feeling.
You can't do this alone.
I suggest you encourage her to see a doctor immediately.
They are seriously nice about the situation.
So many people have the same problem.
Doctors see people every day with depression.
I know when I had problems with it the last thing I wanted to do is tell people.
Like her I didn't trust anyone.
I didn't really think anyone could help me.
When I did finally see a doctor they were so understanding and helpful.
I didn't feel stupid at all.
They will provide her with medication or therapy.
Generally anything she needs to get through this.
If she won't do it for herself.
Ask her to do it for you.
If she won't do it then I am afraid you will have to go against her wishes.
It is in her best interests.
You would actually feel to blame if she did take her own life or do something to harm herself.
So deal with it before you miss your chance.
I know it is difficult.
From my training and from working with people I know depression does not cure itself.
It is a life long struggle in many cases.
Visiting a doctor teaches a person a way to live with the feelings they are having.
Check out some of these website if you need anymore help.
http://www.wikihow.com/Help-a-Friend-with-Depression
http://www.findingoptimism.com/blog/depression/how-to-help-someone-with-depression/
http://www.helpguide.org/mental/living_depressed_person.htm
I really hope this helps x