Could you maybe encourage her to join some sort of out of school activities? That way she would find friends outside of school, friends with similar interests as her, and then even if things get rough at school, she has another group of friends elsewhere that she can talk to.
Also, having extra curricular activities can boost self confidence too.
With regards to her telling her new friends, that is really up to her, i.e. whether she feels comfortable enough, or trusts them, or is confident enough in her own ability to explain them, etc.
I also suggest that maybe her and her mum go and have a chat with a teacher. Teacher's can do a lot to help if they think it is necessary, i.e. maybe doing PSHE lessons on learning disabilities (I am not talking anything like identifying your friends daughter and highlighting her needs, I am talking just a general overview of LDs and how they can affect people socially and academically, etc, and the teacher might also give them projects, or something, to research into it, etc), they might also provide a room where she can go if things are getting too much.
These girls are probably bullying out of both ignorance and fear, and hopefully, if the school can try and educate them, in a general manner, it could help her.
It is unfortunate that in todays society people are still so ignorant that children with SEN are still the targets of bullies (well, that anyone is the target of bullies). In a lot of places the children are now growing up more tolerant and accepting and educated about it. It is such a shame that someone as clearly as intelligent as she is, is subject to pain caused by ignorance.
I really really hope that this starts to change soon and that she can find some decent friends.
good luck to you all