They can't very well say he's real, can they?
Whyever not? I do and have done for years.
Once, when the RE teacher told my grade that Santa wasn't real, I told them (after he left) that he was wrong. I was furious with him for saying it.
In this case, I'd just do the standard, "Well, if you don't believe in him then he doesn't come to your house. So some people think he's not real, but we know better." I would also tell the aide that it's not her place to destroy my child's beliefs with her own and please cease and desist in future.