DD is coming up 12 and has had a friend - previously best friend - who has been making life difficult for the past few years with various forms of excluding DD, such as keeping ‘secrets’/Not inviting or accepting inviatations from DD but will from others, making up extravagant lies to try and outdo DD etc.
Part of the issue is that she then harasses DD if she tries to distance herself I.e. A classic behaviour is that she excludes DD from the group by telling her she’s having ‘a private conversation’, but will then follow her or repeatedly text her and send voice notes asking if she’s ok/why are you upset with me? DD tries to rise above it, and says she’s fine, there’s no problem, but the girl will continue asking until DD admits she was feeling left out, and then berates her saying that DD can’t tell her what she can and can’t do. 
Of course when explaining to an adult/teacher - she’s simply a concerned friend worried that DD is upset......
They’re now at secondary and worst luck, are in the same bloody form and two weeks in, we’ve got the same shit happening.
They walk to school together and DD wants to stop walking with her, but feels bad because this would mean the girl is walking alone (She currently joins DD and DD’s good boy friend - who she says she hates - for the walk).
I don’t like the thought of her walking alone either which frustrates me because I know this girl will drop DD like a brick when she finds a better option.
I strongly suspect this girl will continue to get worse as she gets older, and am wondering if I should request that DD is moved to the other half of the year group - but then DD loses the rest of her friends in the process.
The girl has had a shitty time over the past few years as her parents had a messy break up, which is why we’ve let so much slide, but DD can’t bear the brunt of her unhappiness anymore. It’s just not fair.
How can we phase this girl out and what’s the best way to deal with the hounding that will follow? Has anyone had any success in this kind of situation?
(Just to add detail, I have spoken to her Mum about this in the past who was shocked by the news, attempted to speak her daughter who then became hysterical and refused to go to school. I think she’s happy believing there’s no problem again.)