It's natural to want to protect our kids from hurt. It's natural for kids to want all the latest technology before they have the communications skills to handle it.
Relationships/friendships run their course sometimes. It's okay to let go without working out all the details.
I think you'd be better served to help your daughter develop more self-assurance, more of a backbone, and more independence to handle her own sensitive feelings. That means - sadly - she'd going to get hurt and have some false starts before she learns those skills. She didn't learn to ride a bike or ice skate on the first try - and the process of falling and getting back up is what builds confidence and the ability to move on to tougher things (relationships, academics/professors, and employee/boss interactions). As hard as it is, you have to back off.
That might mean getting her off the "texting instead of talking" train that so many kids are on. They are empowered and emboldened by the distance between themselves and their classmates that the internet and texting provide.
I would have called the parents if the texts were threatening or indicated any kind of illegal activity (drugs, underage sex, nudity) and I'd notify the police or the school resource officer too. I think, beyond that, nasty texts are a judgment call. I would want to know if my kid were nasty to others, but not everyone does - and you knew this mom casually so it's hard to judge what that mom will want. It's also important to judge whether a parent is going to discipline the child who will then make your daughter "pay" for telling on her. I would consider blocking the offending kids' numbers from my daughter's phone (or better yet, have your daughter do the blocking herself!), but otherwise, your daughter needs to learn to shrug some of this stuff off (after a good strong blowout vent at home if necessary) and move on to other friends. If your daughter needs some sessions with a counselor or a life coach to find her emotionally footing, that's a great gift to give her. It's no different than getting a tutor to help with English homework or a sports coach to help with skill building. It really isn't.
Unfortunately, intervening in this case to the other parents tells your daughter that she is too much of a wreck herself to be able to function, and that she's not capable. Doesn't that feed into her feelings of being a victim and being left out?
I'd work with her on what the definition of a friend is - and ask if the behavior of these other girls represent the behavior of real friends. Ask her if she is built up as a person by hanging with people who are so jealous or so cruel.
You cannot stop their communication off line at this point - they go to the same school. You shouldn't call the school and tell the staff to make the other girls be nice to or at least avoid your daughter. That's not their job, unless a real offense has been committed. But that didn't happen here.
You have a middle school child who needs you, not to run her friendships, but to teach and empower her to run them herself. That's why an objective counselor can be so helpful.