I need help!!!!!!!! Plzz read..?

I'm 15, I've a group of 4 girls in school. Out of those two of us are best friends and the other two are just "so-called" best friends. If either of them is absent, the other one starts telling about all the bad the other does and says about us and herself. The other one does the same. It's affecting my and my best friend's friendship also. We don't know which side to go, we're thinking to leave them but one of them is very sensitive, she'll just spurt out tears there and then and repeat this line several time...."I wan't to die", "I want to leave this school"...This has happened before so we're aware, but whenever it comes to tell her that she's not a gud person and all her drawbacks, nothing can explain better than her and she'll begin arguing like anything. So there's no point sitting wid her and talking about this. The other one's a born actoress, she'd talk about the other and add lots of critters to that too. So I never know if she's saying the truth or not. What to do????