Year 5 is an awful time with girls. There are streaks of hormones just starting to kick in.
It sounds that like us, the dynamics in your dDs friendship group/ class is a difficult one. My dD is definitely on the sensitive side although outwardly bright and bubbly. She too does not understand why others feel able to say horrible things to her when she would never dream of saying such hurtful things to anyone else.
She said last week that she just wants them to understand how it feels when someone says those things to you.
I speak to her form teacher if there are incidents that escalate or carry on for a couple of days. In our case it is a very small year group and only a handful of girls. This means that if the bossy girl ( I don't want to use the label bully, I understand she has her own issues) decides that so and so can't play/ join in then no one else can ply with her either. I think the others go along with it as they are afraid if they don't it will be their turn.
It is a roller coaster. Some weeks are awful, with DD not wanting to go into school. (last week)! The teacher has tried various tacks to deal with it. She is aware who is the ringleader, who is the follower, who is the stirrer, who doesn't get involved and who just wants everyone to be friends.
Last week, after trying everything else she advised DD that it was time to stand up for herself. She had been called stupid, ugly and fat. So she told the other girl she had to take a look at herself, as she was the fat one and shouldn't be calling anyone else fat. Cue tears from the other girl, and her other friend falling out with DD because she had been so mean. I don't think that was a good thing to do and said so to DD and her teacher.
Basically, I have no good answers for you. We have tried having different girls for play dates, having the bossy one for play dates (she can be lovely if you're in favour) I try to explain that because A feels bad about herself she says things to make others feel bad and that makes A feel better. But that doesn't stop DD being hurt by all this. I have also said you can't ever get on with everyone in life and that you will always come across people like that.
So far I have held off from talking to the other girls parents. I fear there would be a scene as their DD can do no wrong. The same thing was happening between A and another girl in class. It all got very nasty. That child ended up leaving the school.!
All in all, just reassure as much as you can. Try and show them how the nasty words aren't true, and emphasise the good things to give them confidence.