Parents can sometimes make things much worse, two have in my case by going into the school without me being aware first, and by not having all the facts. I would speak to the Mum.
Start off by saying we all want our kids to be happy. Try to listen 70% and speak 30%, you will come over more calm and in control. See if you could meet in a park etc with her and DD's to see for yourselves they are OK. Sometimes its another child mixing it up for them, making it worse for them. Try to let them to sort it out - outside of school. I asked a mum to do this and she said she was very busy, so i told her I could make it anytime(actually I couldn't) so then she couldn't not accept.
My daughter was told off on 2 occasions in the last 3 years by stories which were made up and very one-sided and escalated by parents, before I had chance to find out . It was not my daughter fault in the end (the teachers even told her that), but she is a quiet girl and wouldn't stand up for herself. My dd has had problems each year and is now in Y6. Same class mates throughout each year, which she gets fed up with. There is no serious bullying like physically hitting - but girls stuff, like being excluded from Birthday parties and the others saying how great it was in front of her (one even told her they were talking about her at a sleep-over) ! Girls can just be nasty.
It is a hard learning curve, but out of this she knows who she can trust, who to be cautious of and those who stay well away from.
TIPS: when she was smaller and someone said something nasty, I told her to image them with elephant ears or a trunk - something that she could imagine making them look really silly. Then to shrug her shoulders and walk off. I also gave her a diary to keep with her in the playground. She could write me notes and feel that I was close by. Or just a nice picture to show me at the end of the day, then she wouldn't be bothered if she wasn't in the gang and left on her own.
She manages much better than ever before and takes things less seriously. She is even friends this year with some of the ones who were nasty last year. "yeah, but that was then she says" and "it doesn't bother me now, I know who I can trust" and that is just one girl in the whole class apparently.
Needless to say the girls are all now friends and some of the parents have fallen out.