Bullying can be insidious and it takes a tough nut to deal with being ostracized. These days it goes beyond school life. Outside school it's exhausting and demoralising coping with public taunts and cyber-bullying. It may be you feel it's too late for DD just to ignore it.
There may be a core of just one or two at the heart of this but as in this instance, bystanders take sides by not stepping in. They take the side of the bully by laughing at DD, encouraging the queen bee by passing on texts or messages on social media sites.
There'll be bystanders who watch or know about the bullying but don’t do anything because they're glad not to be the target.
As far as being involved with the girls beforehand, bitching or whatever, I expect DD was swimming with the sharks, it's a survival tactic to keep the alpha girls on side. Of course that strategy was only working until the queen bee singled her out. It's a nasty backhanded compliment, the bigger the threat your DD poses as a rival, the more that they need to grind her down.
It doesn't surprise me the other parents prefer not to get involved. They know who is responsible, those same names may have done this before on a smaller scale. As long as their DC isn't the target they think, don't rock the boat. I hope they stop at avoiding involvement and not go as far as rumour-mongering and shunning in your community.
Bullying in school can only be squashed if all the staff who have power to initiate change when it's brought to their attention have a resolute zero tolerance policy. Before you burn your bridges at school I would go to the Head again in person and then contact the school governors.