I'm so sorry the parents of Mumsnet appear to be having an extremely slow day. Sometimes they need to be explained to in simpleton terms, very slowly, and loudly. You also need to clap between words.
Allow me:
OP has been supporting another school Mum with disabilities. She has formed a close bond with said Mum, gone out of her way to help her, and the other Mum sends her hundreds of messages a month, because she is disabled and needs help.
The mother with disabilities also has a child in the same class as the OP.
The Mum with disabilities meets the OP, outside of school, their children play with one another in and out of school.
Today, said mother with disabilities has voluntarily offered up information to the OP. This is the following information;
"Hello, OP. My son is having a birthday party/several parties, but you and your son are not invited."
Now, as someone with autism I do understand the need for specifics and pedantics, but it's irrelevant if you're just being a nose-dabbing Karen.
The OP is justified in snubbing this other mother - Disabilities or no. The OP has been used, more than likely because it was cheaper than paying for a carer. The OP has been kind to this woman and this is how she treats the OP and her child.
Yes, OP - You are 100% justified in snubbing this woman. Cut her off. Let the kids play in school, but stop it at that. Don't invite her child to any of your parties- That will be a consequence of his mother's behaviour, not yours. She will create his own losses.
You don't deserve to be treated like dirt after helping this woman. Don't give her a second thought, OP.