This sounds a bit strange to me. Is there something going on in the girls life? I remember a girl in my class at around this age getting very upset when a teacher wanted to make her and her twin sit beside other people. It turned out that her parents were splitting up and the idea of being "separated" from her twin on top of this was all too much. She sounds vulnerable. Hard for your dd though.
My daughter and her best friend are 15. They are joined at the hip and need to be in constant communication by text, facebook, email, facetime, skype - you name it.
She's a lovely girl, but gets upset and tearful if DD can't answer the phone or doesn't text back right away. DD is trying to work out a tactful way to tell her that she can't always answer right away and that if she says she can't talk it means she can't talk.
Also, how do I get them to be less surgically attached to each other and with their lives revolving around when they can meet up?