don't worry about the girl, she's being stupid. just ignore it.
and if you apologize to her... idk
i would say something like: "i don't like that you're dating him, but i don't think we should let it ruin our friendship."
But if she really likes him leave her be, and just aviod him like the plague.
However, at your age girls are going to do what they can to get with the guy they want, it's just one of those things. I wouldn't suggest stopping talking to her if you are in a friendship circle just, try to stay out of her way and only talk to her if you HAVE to, even if it's just a few words, be the mature one here.
As for your ex boyfriend, if you're still good friends with him then what does it matter eh? you aren't loosing a friend here, if this other girl wants to say bad things about you or to you then she really needs to take a good look in the mirror and grow up.
Being called anything, especially 'fat' can tear people up inside so just, hold it together, I don't believe people can be fat, just because they don't look the same as someone else. Fat is just a word used to make someone feel bad, like ugly, nobody is ugly, they just look different to people in magazines.
Hm, I can't really suggest much else unless you make a new friend circle and just block them on IM and ignore the texts, they will get bored eventually. Alternatively, keep the texts and show a teacher what is going on, she might be able to help.
you know how that guy is, and when he screws your friend over, she'll need some support.
also, seeing as she and him hooked up before "going out" i'd say he isn't too into this whole relationship thing. just from the way you described it.
wait it out, and be there for your friend when it falls to pieces.