Hi OP, I have a DD10 and like yours she is also experiencing friendship problems for the first time and it is distressing to observe, isn’t it! My DD is also in a situation of having a formerly good friend become threatened by DD acquiring other friends. I get the sense it is very common at this age.

I think in your case all you can do is encourage and support your DD to ignore BF and move on with her social life. Unless BF is physically bullying your DD I’m not sure what the school can do - they can’t force BF to be kind to your DD. If, as you say, your DD has other friends then hopefully they can help her avoid the exBF.

Unfortunately you can’t prevent all tears, as desperately as I know you would like to as a loving mum. One thing I remember from my own painful pre-adolescence was that knowing my mum was upset on my behalf was far worse than whatever the original hurt was about, so you may want to do everything you can to keep your own emotions in check when you talk about this with your DD.

I hope things get better for your DD.

I'd encourage DD not to bother as much with BF and instead do more things with the other friends she has at school. I'd tell her to be polite with BF but not be as friendly.
I'd tell her not to be bothered about being left out and to persue other things as well, maybe after school and not to let this girl see that she has been upset or bothered either.
I think it will work itself out but its always better to have a few good friends at any age.

We’ve had a similar situation with DD but it was more around Y1. We did exactly what Rudgie suggested. They’re now in Y6 and they’re not best friends but they are still friends and occasionally go to one another’s houses. Once she realised that she could just go and play with another friend if bf was being mean she soon became much happier. We just said things like “perhaps she was having a bad day, if she’s like that again, just go and find someone to okay with who’s having a better day”. I appreciate though that our DD was younger. A Mighty Girl recommends some good books on friendship here

I have the same with my age 10 dd. I they just need to be encouraged to play with other people and not rely on one friend. She also had a similar situation a few years ago where she had a best friend but they were also falling out. That friend moved schools and actually I think it had made their friendship stronger as they don't get sick of each other if you know what I mean. Sometimes I think it's just too much if they see each other every day and out of school as well.