I feel for you. This is horrible for you and your DD.
What I will say is : this wrong un won't change because your daughter has pointed out the injustice of her behaviour. It'll fuel the fire.
Unfortunately we don't live in a nice, fluffy world. There's some wonderful humans about, but there's also a good sprinkling of trouble makers and not so nice /well behaved people around. I think we have to try to teach our kids to navigate those people and circumstances instead of trying to stop those people behaving the way they do. It's kind of preventing your child being on the receiving end.
I have a friend who's child has ASD and they have a strong sense of injustice. She's mollycoddled him quite a bit and unfortunately he's been very much encouraged to "tell the teacher" about any thing he doesn't like. So the poor boy (11yrs old) is now quite the "tell tale/snitch " and "teacher's pet" which of course rubs his peers very much up the wrong way and ends up making him a target. I cringe , because I think, in adulthood there is no "teacher to tell" who's going to intervene and tell everyone to use "kind words". That's where you've got to learn (and it's hard ) the skills to navigate difficult people
What I've taught mine is to rub along with the "roughers" as well call them (yeh yeh , I know) because it's less likely to make you a target. Don't have to be best mates, don't get too close, but rub along with them.
Your DD probably should have picked her battles. (It's tough and I feel for her ) I get why you confided in another parent, it's allyship and being vulnerable and alone with the burden of it. But you have to be a bit guarded with people you don't know too well. You don't know if you can trust them, and you made an error of judgement.
I'm afraid I don't have any sound advice really on what to do next , but I would try not to respond too much to messages: let it die down. It'll hopefully run out of steam. It needs reporting to school in this instance though, as it's already gone too far. I'm sorry you're going through this.