She needs to be with an instructor who is trained to deal with this. In other words, tell your boss and they need to direct her to the correct facility. Not to mention this could be a safety issue.
I know, that in many towns, there isn't a "correct facility". Parents end up gym hopping hoping to find someone that will work with their child. I think trying to come up with some solutions first, that might help is key.
I mean, we have all seen kiddos on their first month of gym class that struggle. It is loud, busy, hot, chalky, and lots to do. Sheesh, most boys will want to just run around. It is how the coaches deal with it that will determine how it goes. She may need a more black and white thing, like a circle or carpet square to sit on utnil it is her turn. She may need to go to smaller class. We don't know. The key is to get the mom and supervisor on board, hopefully the mom first.
If mom approaches you again like that,you could say "yes, I have noticed that, but I do need to work with the whole class. Do you have any suggestions for what works for Suzie so that I can help her best learn?" If mom says 1:1, then you suggest private lessons, and give her your available times.
If you talk to the supervisor, please do so for ideas, or help in class. Please don't have someone that hasn't seen and does not work with the child talk to the mom first.
It could be any number of things, that could be a disability of sorts, could be that she is spoiled, could be that she has had a major life event happen and she isn't coping. IT could just be that she has never been in an environment like this.
Good luck! I do know how hard it is. In addition to having a child with special needs (that is now in AP Classes in high school, even with the "issues") i teach children with special needs. It can be tough. but like I said, we don't even know if this is the case with this little girl