Let your child know that there are certain instances where she should never try to intervene, as it might be too dangerous. If the bully is mentally unstable, if there is a weapon involved, if there are multiple bullies, or if the bully is violent, instruct your child to go get adult help immediately.
There can be ongoing discussions on how to prevent kids from being inadvertently socialized into bullies, instructions on how to bully-proof and raise the self-esteem and confidence of your children, and warnings about the potentially destructive nature or untapped positive potential of bystanders.
Fact
The outcry from the murder of Kitty Genovese in 1964 (in which dozens of witnesses heard the attack, but no one helped the dying woman or called the police) uncovered a psychological phenomenon called the Genovese Syndrome or Bystander Effect. Simply put, as the number of witnesses to a misdeed or crime increases, the chance that anyone will intervene goes down. Most people assume that someone else will help, and tragically, as in Kitty's case, no one does.
But ultimately, there is only one permanent way to stop the phenomenon of children hurting other children, and that is to establish a social climate where aggressive and abusive behavior is not tolerated.