Hi L.,
It may not be every girls memory of being a young girl, but I remember what girls were like. It is in the nature of girls to pair up. While boys are more comfortable in groups, girls tend to find a "bestie". In young girls and all the way through junior high or middle school (and sometimes highschool) it is high drama. It's almost like a relationship with breaking up and making up. There is cheating, betrayal and jealousy. It's difficult to watch. We want our kids to branch out and have a whole herd of friends and our girls came sometimes tend to narrow there world to this one person. So hearing you say that your daughter says similar things about Sue doesn't surprise. Girls crave intimacy and bonds with others.
As to the mean or jealous part. Girls can be like rabid wolverines. There is always a top dog and then there are the underlings. There are the underlings in the circle and they are treated slightly better than the underlings out of the circle. One day you are "in" and the next day you wore the wrong shoes or everyone was wearing a yellow shirt but you and you are "out." It can literally become an eat or be eaten mentallity. If your friend or "bestie" or the head chic doesn't like that girl and you befriend her then you are out and you find yourself in the same boat she is. You are teased, ostracized, made fun of and you may never enter the circle again. If you are going to be going to school together for the rest of your education, that can mean 8 or 9 years of pure misery. Your daughter has probably experienced some of that and may be starting to geel like she doesn't want to be eaten. Most would choose to be strong rather than weak and the unpopular girls are considered weak.
It is really important, I think, to instill our girls with a sense of self. We need to make sure they know and like who they are. I would start making sure boundaries are set between your daughter and Sue so that your daughter can clearly define where Sue ends and she begins. I would talk to her about the dynamics in school, if there are any mean girls, how those girls acts, what she doesn't like about it and how she might want to be different. I would get her involved in activities (girl scouts and such) where she can make many different kinds of friends so that her world can't be narrowed to what a few girls at school think or to just one friend, so that it is easier to go her own way.
As to discipline, I would make sure she knew that if she could not make herself be kind, that she would be grounded. If she doesn't treat people well, then she won't be around people.
Sorry this is so long, but this is something I feel passionately about. We need to build our girls up so that mean girls don't really matter so much and they dont feel the pressure to conform.
L.