B.H.
I've been a parent for 27 years and an elementary teacher for 25, and the advice others have written about caution in ordering her not to see this friend are dead on.
Our job as parents is to instill values so that the child can learn to respond appropriately to all kinds of people. Keep repeating (as trite as it is) the Golden Rule. When you're snuggling with her before bedtime, let her know how proud you've always been of her careful heart, that it's one of the things that makes her special. And share your desire that she would never outgrow that. Unless there is physical damage occurring, parental intervention in childhood politics is dicey. Your daughter needs to learn to trust her own abilities to behave appropriately. Keep affirming the positive values, and she will come to embrace them as her own.