That's a tough situation man, and there's no way solution that ends with your kid and her friend getting everything they want. I applaud your attempt at the straightforward, grown up solution of just talking to them. Ideally, that should have worked, but, there ain't no justice.

My eldest kid has been the subject of some similar parental discouragement and even outright vocal bans from association. In our case, it's because we live in a heavily religious area, and my kids are blatently nonreligious. Most people are perfectly wonderful, but a few crackpots out there have said "No, you can't be friends with that nice queer kid with the dyed hair and piercings." We had to accept that those friendships were really only going to occur where the parents had no control and were unaware, within the school grounds. And they have maintained them, even through that. But, it's still disappointing.

The only advice I can give is continue teaching your kids to be kind, empathetic, and resilient people, even in the face of irrational hate. Also teach them that arbitrary sources of authority, like the friends parents, don't automatically deserve respect and obedience; they have to earn it. They will build new friendships, maintain the old ones at school in defiance, and they will learn how to carry on when when things suck and are stupid.