You’ve done a wonderful job of educating your daughter about the meaning of true friendship. Naturally, you want to protect her from hurtful or abusive relationships and you want to protect her fragile self-esteem. You notice, however, that she keeps going back for more trouble, because she can’t seem to set her own boundaries. Your job is not so much to solve her problems for her (by intervening with the “mean” child) but to help her build the skill of boundary-setting.
    The first way you can do this is to model it for her. You must, painful as it is, set a boundary between you and your daughter. Now that you’ve given her appropriate guidance, you must let her know that this problem is her problem—not yours. You’ve told her what she needs to do about it if she wants to solve it. Now, it’s up to her to try out your advice. When she doesn’t choose to try it and instead comes crying to you, you need to set your boundary again: “I’ve told you what you can do about your problem, and now it’s time for you to do it.” Do not give her lots of sympathy for the continued problem that SHE is not dealing with. Just respond in a calm, unemotional, matter-of-fact way to her complaints. Don’t get involved! Don’t show that this is affecting you, since it is important to show her that this is HER pain, not yours.
     Even though your daughter has the poor social skills that some kids with ADD and LD have, she is still capable of learning new skills. It may take her longer, but she’ll get there if you make it clear to her that she is supposed to! Under no circumstances should you take this problem on, as if it is yours. Do not try to solve it for her. This sort of enmeshment shows your daughter that there are no boundaries in relationships—the very opposite of the lesson she needs to learn.  By standing back and asking her to take care of the situation, you give her the impression that you believe in her capacity to handle relationships. This message is an important one for her. She needs your confidence. You need to be patient, because it may take a while longer before she decides she’s had enough with this “friend.” However, if you step back, you will speed up that process. Once she sees that no one will rescue her, she may decide to rescue herself.
Watching our kids suffer is the very hardest part of parenting. But we sometimes help them the most by allowing them to go through their own process and work things out for themselves.

I agree with the previous poster on many things - however, I do believe that the girls' parent(s) should be spoken to again about their daughter's behavior  - I don't think that showing your daughter that you are sticking up for her in some way will damage her coping skills - it may just help her confidence to know that she is being helped in some way, and may serve to "disempower" the bully just a little bit - which surely would not be a bad thing.    You should not feel guilty at all about defending your daughter.