Facebook is great. Its a tool that allows people to explore their unbridled narcissism and literally have a one-sided conversation with all of their friends at once.
Thanks to Facebook I've learned who is shallow, stupid, homophobic, religious, ignorant, and gullible. I've learned who I need to unfriend both on Facebook and in real life.
People judge you by the company you keep. Its unfortunate, but true. If you're not able to simply ignore what they say then maybe you should drop them from your friends list.
Likely you telling her that shes behaving obnoxiously will be treated as an attack on her, or a criticism. She'll feel you're acting out of jealousy and not of compassion for a friend. Her self absorbed attitude won't let her believe that someone is acting out of her best interests because in her world, she is the nucleus. Everyone surrounds her and exists only for her needs.
The best case scenario is to ride it out until she grows out of this phase.