Emmabanana1
So basically I lost a friend after I got worried because she keeps starving herself.
I snapped at her, and she basically ditched me.
I apologized, but she didn't accept and sided with someone who really hurt me.
What should I do? Should I apologize again?
I can understand why you snapped; your friend doesn't seem to know how good of a friend you are. You snapped because you were concerned for her health, and you're in the right here. Hell, I would've done the same thing! But if she keeps starving herself, then it sounds like she's got an issue that her parents need to know about; your friend needs help. But the fact that you offered her help and told her the consequences of her action through a snap, she took the concern the wrong way and sided with said person.
I suggest you talk things with her and tell her that you acted the way you did because you were very concerned for her, but also ask her why she left you for someone who hurt you. You might also have to ask her why she's starving herself and acting this way, because there is something bigger in the picture here that's causing her to act like this, and as a friend you need to know. Her parents need to know too!
But just like kitty said above me: if nothing is changing, I suggest moving on and finding a new friend, because it's obvious the girl is quite insecure and immature.