My best friend

My best friend always seems depressed when I’m not around. When I’m with her she is all happy and everything always falls into place for her. She gets all the guys, has million of friends, and gets whatever she wants. But when I’m not with her she is always sending me things like she is depressed, her diary entry’s(which are pretty depressing), her crying, or that she wants to die. One time she took pills but only 2 or 3 just to show people she would do it. I have no idea what to do. I want to help her but I don’t know how. Can anyone give me advice??