Prepared to be told that I'm BU as I'm very emotionally invested in this and sorry for the essay in advance
When DD1 was at primary, she had a problem right from the beginning with a particular girl. Just silly things like "I don't like [DD]'s hair today" or saying she couldn't play with them. It was very childish and silly but they were children and it was impacting DD quite a lot so eventually we had a word with the school. They had a whole class discussion about being nice with each other and promised to keep a better eye on DD, especially when interacting with this girl, which satisfied us then.
A couple of weeks later DD told us that her teacher (who had kept her promise) had witnessed a nasty comment and reprimanded the girl. From then on DD didn't complain to us anymore and we assumed (very wrongly) that it had just taken a sharp word to sort the girl out. DH and I know now that this was very naïve, please don't flame me for our past handling of this because I want to focus on what's currently going on.
What essentially happened is the girl became very sneaky in the way that she would make these comments, and make them privately to DD when teachers and other classmates wouldn't hear them. She also repeatedly told DD, "nobody likes a tell-tale" etc. as well as other people in the class, which created a culture within their year group as hating "snitching". It caused problems for other students as well but mainly DD, who was very anxious/sensitive and so stopped telling us when the girl made remarks. She also did things like conditionally allowing DD to sit or play with her, based on her "behaviour" that day, so DD was desperate for her approval.
Over the years these comments escalated massively from nasty to absolutely vile. They brought me to tears the first time I heard them. DD went through a period of anxiety, paranoia and just self-hatred over Years 4 & 5, and part of Year 6, but wouldn't tell us where it wasn't coming from. Then one day a boy overheard her talking to my DD. He was upset by the comments and told his father, who reported it to the school and they "investigated further" but not much was done. Until this girl's mother marched up to my DD in the playground and very loudly asked her "why are you making up lies about [her daughter]?" DD said she wasn't telling lies, not even assertively, it was more like a plead, and then the mother began screaming and raging, shouting abuse. She actually looked unhinged. My very anxious and sensitive DD burst into tears and after that the school took it much more seriously.
It turned out that as well as the absolutely vile remarks (which I still don't know where she got, they were repulsive), the girl was doing things like ruining DD's clothes and hiding her things in the classroom. This was about four weeks before the end of term before it came to light, so while the school kept them separated and monitored (and made the girl apologise 
), not much was done as they were leaving, to different schools. DD hasn't had contact with this girl since but it still took her years and counselling sessions at her new school to be open about her emotions and say when something bothered her. Her anxiety/paranoia lessened some years ago but has got worse Year 11.
A week ago this girl sent a friend request on Instagram. Neither DH nor I were happy about this at all, but DD looked very happy with it. It just reminded me of little her overjoyed that she was allowing DD to play with her today 
But DD very much wanted to do it - said they hadn't had any contact in 5 years, it was just seeing each others photos, she wanted to put it behind her etc. etc. We agreed but on the condition that DD told us anything that happened and if it began making her anxious/uncomfortable she was to block the girl straight away.
Two days ago DD posted a photo and the girl commented "hmm." DD immediately became very anxious about it, and eventually took the photo down because it upset her so much. DH and I said she was to block the girl and DD agreed it would be best for her.
This evening a girl that was in their class sent DD a screenshot. A girl had posted a silly mugshot of herself and the girl had commented "you look like [DD]". Another friend that DD used to school with commented that it was unkind, which helped a little, but DD was still horrendously upset She hates the idea of this girl making comments about her and is worried it means people won't like her anymore! She talks to literally the one girl who defended her, but is still upset at the thought that they wouldn't like her. It's awful and I don't want this nightmare back in our lives.
DH says DD should make it clear to the friend that DD doesn't want or need any more screenshots, and DD needs some help (possibly professional) at literally just forgetting this nasty girl. We had a nice spa day together to take her mind off of it, DH made her favourite dinner for tea and now she's watching a bit of TV with DS before bed. She seems a lot happier but the more I think about it I want some actual action taken. I'm just so upset on her behalf and don't want this to escalate again.
AIBU to want to do more than simply remove this girl from DD's life? I don't want to make it into something bigger than it is and stress DD out, and I don't know what we could even do. I'm not sure if I'm just out for petty revenge or punishment for this girl because at this point I've lost any objectivity, but the more I think about it the more I want something more to be done.