How do I handle this 'frenemy' situation

[oywiththepoodlesalreadyy]

My dd is 8. She has a best friend since nursery. They get one pretty well from what I have seen and I'm relatively close with her mum.
There have been issues in the past where her friend has been manipulative or lied and I've just brushed them off, as has my dd.
Last week, dd got upset before school, said she didn't want to go because she was too ugly. It was so out of nowhere. I tried to talk to her but she's very difficult to have a proper conversation with (she has adhd) and so reassured her and off she went to school.
Before bed that night I said we needed to talk about it and we eventually agreed we would do this by writing instead of talking (worked a treat!) I asked if someone had told her she was ugly and she wrote yes. She then wrote that she didn't know who. So after further discussion it transpired that she didn't hear them herself. Her 'best friend' ran over to her to tell her that some kids on the playground were saying how ugly she was. And as past behaviour dictates I knew she had made this up to upset my dd. So I told her that didn't happen.
She then opened up and said she thinks she knows why she said it, because she had been playing with another friend and her best friend never likes to play with anyone else nor want me dd to play with anyone else.
The fact that her best friend had her looking at herself in the mirror and thinking she was ugly absolutely broke my heart.
I had a conversation with dd about jealousy, and how it's a reflection of her friend feeling jealous/upset/insecure etc. and she then went on to tell me a few other things her 'best friend' had said.

They sit next to each other in class and I've told my dd I can arrange for that to change if she wants it to without it being known she asked and she said she will think about jt.

Dd is struggling in school and at home at the moment emotionally and I just don't know if I'm handling this right? If i could do more? Do less? Should I mention to the teacher etc?

Unsure how I can raise this with her friends mum without me out right accusing her daughter of lying.

Don't talk to the mum. Just talk to school, say their relationship is causing your DD to feel upset and worried about various things and you'd like them to be moved apart and supported in nurturing other friendships.

Trust me - been there, done that. Its very straightforward but feels like a big deal at the time.

And keep communicating with your DD in the way that works for you. This phase will pass.

P.S. all of 'who said what' is irrelevant, don't dwell on that with the school. Just stick to the basic principle that they need to make other friendships for now.

Yes I would raise it with teacher, in confidence, so the teacher is aware of the situation and can if needed check in with both girls.

I would also ask if the teacher can please send Dd out to play together with another child for a while to build her confidence

I wouldn’t speak about it to the Mum, as if you give the teacher time to assess, and your Dd starts to out to play with others, it will settle down.