Looking at this from the perspective of being a teacher, It would be better to have a parent in the classroom to assist, than no one at all. A high special needs students who does not have an assistant, takes alot of time away from the learning of other students, it is unfortunate, but that is the way it is. So to have an extra pair of hands is invaluable, believe me I have been in many classrooms with special needs kids (although probably not as high level) and no assitant and it is hard work for everyone.
As a parent as a high needs child, currently going through the insane guidelines of funding, I can totally agree. There are many classroom aides at DS's school and each one is amazing in their understanding and generosity of spirit. I have found that many of these aides have special needs children themselves, have completed a short course (and police checks etc) to give back iykwim?

Do you know exactly how 'unqualified' this woman is? I TOTALLY understand your concerns - but it might just be a matter of her volunteering her time as she gets her quals. I would imagine she is attracted to this position because she would like to help kids.....and in a world that most parents can't spare the time to help in the classroom, ANY help is wonderful.

I think it would be a better idea that the school sends a letter home to the parents formally introducing this woman rather that just sticking her in the classroom and leaving you all to wonder!