First of all I'd want a screenshot before I'd take anything as fact. No offence to your friends but stories tend to grow arms and legs in the telling, so I'd want to know exactly what was said and in what context.
I'd also want to speak to the school to find out exactly what happened and why you weren't contacted. I can't help but think it can't have been that serious if you weren't called in? DS1s school (secondary) calls if there's been anything over a bit of niggling over a few days.
If she has been slating your DD on a group text then I think you've got every right to ask her to stop, and to keep communication about any issues between herself and the school. Because let's face it, it's a nasty piece of work that gets off on slating a child (irrespective of behaviour!) to other adults.
There's a wee girl in DDs class whose behaviour is pretty wild (huge backstory, neglect, abuse, now in care) and I was horrified to hear a group of mothers in the playground talking about her the way they'd slag off an adult (name calling, swearing, the works).
I fairly bluntly said that she's 5 years old, and that their behaviour was far worse than anything she'd done because they're supposed to be bloody adults!