I think the difficulty is the interpretation of it all.
As takeiteasy says, the 'taking her best friend off her' is often how it feels, but it might be wise to find a way to spin it so it's less difficult.
I had a similar situation at the beginning of Y1 with my girl and her friendship group. I admit this was easier to manage because of them all being five, but I had the same situation with DD coming home having been told that she's not allowed to play with a couple of girls (B and C). B&C wanted to play with A, DD's best friend, but not her. A, being five, revelled in the attention and quite liked being fought over. B&C, also being five, enjoyed the attention and power.
The way I explained it to DD was that A still loved her, but sometimes in life, people want to get to know other people and perhaps spend some time with them. It doesn't mean she loves DD less - it just means she's curious about all the other people in the world.
The way for DD to deal with it, is to take the time to also explore all the other children. I did encourage this by organising playdates with some of the others, and it turns out that DD is popular and there are lots of cool girls in her class whose company she really enjoys.
As it happens, when A noticed that DD was not that bothered by her absence, she suddenly realised she wanted to join in with the cool girls too, and now the two of them are back to being inseparable, but they're also part of a much wider group of children. This brings the bonus of me having a wider group too. I have to admit, I have no clue how B&C are fairing, but that's their mother's issue rather than mine.
So basically, it wasn't nice in the few days when it all kicked off, but it's ended up being something really positive.
The actual crux of the issue;
my DD and her Best Friend had apparently been upsetting Girl C and made her cry calling her mum a liar that she had not been able to talk the Best Friend. They were also shouting in her face. Girl C had before this been telling my DD and her Best Friend that she doesn't like our families. my DD and her Best Friend had apparently been upsetting Girl C and made her cry calling her mum a liar that she had not been able to talk the Best Friend. They were also shouting in her face. Girl C had before this been telling my DD and her Best Friend that she doesn't like our families.
is something different. Girl C may have said something unpleasant, but your daughter and her friend closed ranks and overreacted to it.
From Girl C's point of view, there may be a very different story. She might not be trying to take DD's best friend, but trying to make a close friend. She might not see it that Best Friend is 'spoken for'. Yes, how she's going about it is clumsy, but she's nine.
From her point of view, it may be much more that she was trying to make a friend or two, she was making headway with this, then suddenly it all started unravelling and in a panic she retreated to being mean, which was then followed by her 'friend' ganging up with another girl and screaming in her face.