ToffeeCake!
You're weird. You say:
"you assume that whoever told A made the group seem nastier"
There's no assumption happening at all in drawing a conclusion from a piece of evidence, in this case the behaviour of a person. If someone steals they are a thief. I would not be assuming they were, if they stole something the evidence says they are a thief. No "assuming".
The person who told A, did tell her! Simple as that. No assumptions! Someone told A. A knows because a person told her.
Is that clear enough for you? Yes? No need to assume someone told her then. Whoever told her knew it would cause a lot of trouble. A colloquial term for a lot of trouble is shit. So the person did stir up shit. It is obvious s/he knew it would ensue from telling A. That was the reason for telling A. A real friend would have taken the group up on whatever they said there and then and challenged them on the spot.
I feel sorry that the OP hasn't got across to you people that they were not being mean about A except for one person's "confession" and that applied to one person.
I am appalled at the low level of ability to read a narrative and understand the facts set out in it which is revealed her on this thread. The desire to jump in with both feet and mercilessly attack somebody from a great height of higher moral ground is overwhelming.
The lesson is; if you have something you need help with, concerning relationships especially, do not go to MumsNet.
I would have told A to stop speaking to me like that because I had not said anything to hurt her. And the fact that she jumped to the conclusion that they all were hurting her, must be because the person who told her about it said they were. The way she went for them and, worse, attacked the 14 yr old, sounds like somebody who likes a fight and doesn't bother to get the facts straight first before she sets into people.