OP, you are worrying about how to fix it, but you don't have to do anything. You don't have to oblige nor explain any further. You've already started by avoiding her and not letting you nor DD be railroaded. I wouldn't say anymore as you don't want this to escalate. She hasn't listenned and is unlikely to unless you get loud about it and I think that's a mistake in the long term. Just see your reply as a response to her shitty text and leave it at that.
If it helps- I had a similar issue when DD3 was Year1-2, with Other DD (both DD3s) whom she knew from babies. And I'm glad I dealt with it quietly without any big fall out.
By halfway through Yr1 DD kept talking about how OtherDD was mean to her (calling her "stupid, weird, mental"), bossed her around, made her play her games only, sent other friends away, and stamped on DD's foot, kicked her or pinched her on carpet time or in classroom if DD hadn't played with her at lunchtime.
Both DDs were originally popular but OtherDD became less popular as time went on because she was mean sometimes. My DD said OtherDD told her she had to play with her as they were "best friends". DD didn't want to play with her. She didn't even like her by that point.
I quietly asked teacher to move her place on carpet and table away from otherDD. (Teacher had started to notice my DD being teary or moving away on carpet).
I chatted with DD that she could choose who she played with or was friends with. And she didn't have to spend her playtime with DCs that hurt, bullied or made her feel bad about herself, as true friends are kind and thoughtful to each other.
Other Mum kept texting me and cornering me at pickups insisting I made my DD play with hers, as OtherDD "had no one to play with" (not true, + also as school has friendship bench and older pupils- play leaders- who take alone DCs into big group games) and that we should meet up /playdate so they were better friends again. My confident DD just said no thanks mum.
I refused to get drawn in by OtherMum, ignoring messages for days then late replying, we were always busy, changing our plans, and I replied noncommittally when put on spot by OtherMum at school gate "oh, i think the girls can choose who they want to play with, DD says OtherDD plays with other friends... Children drift in and out of friendships over time...best not to interfere" (No way was I giving her ammunition or anything to latch onto)
OtherDDs had plenty of friends but they hit her back if she hurt them, or told on her. My DD didn't.
DD said OtherDD was on sad face for being unkind (hurting) quite often. DD simply went off and played with others. (DD was only even in safe face for talking too much in class 🙊)
We became unavailable for playdates outside school. Both developed other friendship groups
in school & outside naturally.
Years later both DDs still play together occasionally in groups at lunchtime, are friendly enough in school (no outside playdates, they say hi if bump into each other at same place or party) and DD walks away if OtherDD crosses her line as do her other friends. My DD learnt a strong message about real friendship, that will protect her in future.
But as there was no big falling out nor confrontation, it left things open should she grow out if it, or if they want to be friendly in future.
Also DD will be less likely a target of OtherDD in secondary school, who whilst no longer physical, can have quite a mean tongue on her. Both because of DD having other strong friendship groups and that we didn't make enemies of either otherDD or OtherMum (as Mum can be mean too, not really intentionally). Important when our DCs are likely to go to school together for 13 years, as it's the long game you play. So I recommend avoid confrontation and subtly withdraw without burning any bridges but not engaging with any stalker shit.
It'd be so different if it was full-on outrageous targeted vicious bullying -that you have to get school to act upon/ avoid outside school), but at age 6 it's often more testing the ground subtle stuff, that may or may not last.