Children learn by example AND BY NO OTHER WAY. The fault of this problem lies with the girl's parents.
That said, who is the real victim? And who will be the more moral person?
It falls to us, those with higher morals, to suffer at the hands of those of lesser repute; and then to literally turn the other cheek.
No, that isn't just; that isn't right... but it is what must be done. Too many people "seek for justice" when actually they want revenge. I seriously doubt that you are out for revenge but are feeling pressured by that many-tentacled beast called "Society". It's this same Society and "Public Morals" that has spurred Popes to declare war and fuel endlessly the Spanish Inquisition. In the Name of Righteousness, we have people parading around with signs that say, "God hates fags" and burning crosses on front lawns.
I am personally heart-broken that this happened to your child. I have raised three children and they all have had their moments wherein injustice was dealt to them. It happened to me as a child and it left me morally distraught. It still happens to me as I try to obey traffic laws as I drive.
The story of Jesus does give us a proper example. We learn by example.
Is it easy or convenient to do what is right? Not at all. However, I will not budge on my obedience to the law to accommodate law-breaking drivers. Yet those law-breaking citizens constitute "Society". Am I fighting against society? Well, I guess so.
I would gladly come over to your home and demonstrate by my example that it is possible yet painful to do the right thing. I have the fruits of my labor to prove my case. All three of our children are in their twenties. Not one of them has ever done a single solitary thing that has brought grief to us, their parents. They all wish to live nearby us; two of them are able to do that. The middle daughter has been out on her own since she was 17. She has every opportunity to do whatever she wished with her life. Yet now that she has found a guy who loves her for who she is, her next heartfelt wish is to live near to us.
That's what I want for you. If, when your daughter has reached adulthood, she desires the most in her heart to remain close to you for your support, love, and trust, what more could you want? Would you really want "justice" so much as Society would have you believe? I don't believe so.
I hope that these thoughts become yours and that you share them BY EXAMPLE to your daughter and to the offending friend and to the offending parents of that child. Will the offenders respond favorably? Very likely they will not. And once again, the fault lays with them. You will have done the right thing. And I sincerely believe that the entire experience will draw your daughter closer to you.
Please excuse the length of this answer. However, the subject is of paramount importance if we are ever going to consider ourselves civilized human beings.
These are my true feelings. And as a side note, did it matter at all that I am an atheist?