Just awful behaviour.
That child is not nice.
Nice children don't do that.
I think whilst it is upsetting, there is nothing to be gained from saying anything but the truth.
That you too are surprised and clearly she isn't as nice or as fond of her as you thought.
I would advise your daughter to focus on other relationships and suit herself.
Self respect is important.
We teach people how to treat us and it is important for her to learn that she doesn't have to accept behaviour that she doesn't like.
Years ago something similar happened in my daughters class age 10.
The little girl was very hurt.
Her mother was very blasé about it, girls will be girls etc..
As it happened this hurt childs mother brought them to activities 3 days a week🙄after school, and her daughter said she didn't want that to happen any longer.
Her mother sent a text and said whilst she absolutely accepted that her daughter had no entitlement to be invited, she no longer wished to provide lifts.
She then received an invitation which they declined.
There was murder and thats when I heard about it.
The mother described it as very petty etc.
The parents that spoke about it to me had zero sympathy for mother or daughter and said it was a lesson in consequences.
The mother had clearly, conveniently, forgotten just how obliging the mother had been.
I had zero sympathy for her and thought they were absolutely right to halt the arrangement if that was what her daughter wanted.
As the mum said, she hadn't a notion of having an atmosphere in the car and her priority was her daughter, and she didn't want her in the car anymore.
In this situation I blamed the mother 100%.
She should have pointed out the reality that she was getting lifts 3 times a week and there might be upset and consequences.
Her mother really fxxked up IMO and she was unable to continue the activities.
As is often the case the girls got over it within a couple of weeks because they were in school together but were no longer close.