I don't think her selection of friends is really an issue but more how does she interact with them? And then on top of this is her way of interacting and selection of her friends slowing her down in her abilities? If it isn't I would not consider her friend selection a negative at all. In fact it could be a sign that she is a well adjusted child and a major positive.
But I think your concern is more if you do accelerate her will she suffer do to the fact that she will be the youngest in the class and won't have her usual stats of friends to pick from? I think children are residual and capable of adapting so if she is showing signs of boredom in school and not being taught then this just might out way her friendships. Really is a parent's call.
