I really feel for you here. I can see this from both sides. My elder daughter sounds a bit like yours, but by Y6 she was trying to ‘fit in’ a bit more.
She had a friend called Poppy who was quite eccentric, lovely company for an adult, but actually quite hard work and demanding as a friend. Very controlling. I’m a clinician working with autistic children and I’d say Poppy was certainly on the autism spectrum. Over the years my dd was very accommodating to Poppy.
When my DD began to branch out socially, she had to learn to assert herself. This included telling Poppy that she didn’t want to do something (Poppy wanted her to partner up for world book day, my daughter wanted to do her own thing).
Seemingly Poppy took offence, told her mum that dd was turning her back on her. The mum (who is lovely) cornered me at a school assembly and told me how desperately hurt she and Poppy were that my DD effectively wouldn’t do what Poppy wanted. Said they’d been friends since reception and now Poppy felt she was losing her only real friend. It was a quite an emotional burden for dd as they were doing SATS.
There was another occasion where Poppy was laughed at by the boys in class for having to walk to school the long way round down the path rather than across a field with everyone else. The field was muddy, Poppy’s mum wouldn’t let her walk that way unless she took wellies. I always felt she should have let her get her shoes muddy, it would have saved a lot of trouble. Poppy’s mum was upset again with my daughter for not going ‘the long way round’ so Poppy wasn’t alone. My dd was just trying to fit in.
Inevitably, dd and Poppy grew apart. It was too much pressure. I look back and realise that dd tended to have a lot of friends who were shy, or a bit different, probably because she was shy herself, and also gentle and accommodating. I also realise how much the mums of those girls had come to view my daughter as their child’s social ‘helper’ because she could keep them ‘in’ with the wider social group in school.
So I do see this from both sides, and when you mentioned her ‘latching on’ to another child, I thought how this is being experienced from that child’s perspective. I’m sure your dd isn’t anything like Poppy, by the way, I’m just thinking I probably wouldn’t do anything active about this, rather I’d try to encourage dd to join in with more mainstream Y6 activities, even if she doesn’t like them. I hate social media, but I have to admit that my girls would have been left behind socially without it. I have always screened their use.
I hope this reply doesn’t sound harsh, it’s not meant to. Y6-y9 is awful for girls I think. Hopefully she will find her tribe at high school