I have a 9 year old daughter and have gone through the same thing. My daughter struggles to read and they do reading buddies with the grade 5 kids.
My daughter told me once that the older girl supposed to be helping her was getting angry that she struggled with certain words. We had the chat about bullies and how to handle them. Ignore them, state clearly to stop and not accept the behaviour and if all else fails, see the teacher immediately.
She told the teacher the next time it happened and the teacher did nothing! Then I heard the girl grabs her wrists and told her to LISTEN.
I went to see the teacher personally and the other child's teacher. Demanded that there be some consequence and to remove my child and the other girl from the reading buddy system.
Sometimes you just have to get involved personally and make sure you are your child's advocate because nobody else will!
(Hugs)