This is not easy, and it is hard to see a child struggle.
First, you indicated that the child has been tested for dyslexia. I would get in touch with a well regarded speech therapist and ask for a learning eval...just to rule things out.
Second, if the mother has any friendships with the parents of other children in the class, ask if their child has made any comments about the girl in class.
Third, listen to what other mom's describe the classroom setting to be. Get this girl in a nonthreatening environment, and ask her if she likes school, her classmates, her teacher. What she likes and dislikes, and listen very very carefully.
Two lessons we/I have experienced. My son did not want to add to our burdens as we were dealing with some health issues with his younger brother. He was a) bored, b)being bullied by some other children, c) had a teacher who was not competent...a good teacher does not mark work wrong when the child includes information that is correct but not yet presented in the classroom, only to mark it correct the next week when the teacher presents it to the class....a smart child will see through such stupidity on the teacher's part and ignore the teacher and other children who are behaving badly. After dealing with a son who tuned out in class, did not pay attention, had stopped laughing and smiling at school, (we had a very unhappy son by that time)and a teacher who was not teaching well...one doesn't pull that marking stunt on 3rd graders, and one doesn't verbally tell a class of 3rd graders what the homework is, telling them to put it down on the planner, and doesn't write it on the board or check the planners to make sure the data is written down correctly. When we got our son out of his nicely expensive private parochial school into a private, small, well monitored and instructed montessori school, he finally started opening up about what was going on and my heart broke week after week as he started letting us know just how ugly an environment he was in in a very Christian school. Well, Christian is as Christian does, and his montessori school is much more Christian in a nondenominational way than that other Christian school....my son is back to being happy, smiling, laughing, learning, eager, we are so lucky.
The second story is about myself...I was in the 4th grade, and my teacher was ugly. She yelled, and threw chalk at students. I hated her. So much so that I started to ignore her...she was rude and ugly to everyone, and if I ignored her like I ignored bratty children, she would go away and I wouldn't have to deal with her ugliness in my world. So, because I obviously couldn't see or hear I was moved to the front of the room. When I didn't do my work I was sent to a doctor to have my vision and hearing tested. The doctor asked me what was happening at school that was causing all this problem, and I told him. My mother hauled me back to the principal's office and made we tell him everything, and demanded I be moved from that teacher's classes, and into a more nurturing classroom. Well, in my new class with my wonderful teacher I had to do all the work I hadn't done for 4 months..which was a pain but my teacher was nice and sweet and made me want to learn for her...the ugly one's contract was not renewed and no more chalk was thrown at students in school.
So, the environment makes the difference. I suspect this girl is intellectually smart, but lacks the confidence and social skills to manage what may being happening in the classroom and school environment. If she ignores it, it can't bother her. This is not going to be fun or easy to address...but this girl has got to know that you and her mother are on her side, and belive her no matter what. If she won't discuss what is going on, get her out of that school. It is toxic.