I've been in a similar situation a couple of times before, and I always make a point to apologize directly to the child for their parent's 'terrible behavior'. That usually gets the shitty parent really riled up, which is always good for a laugh.
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There aren't a ton of great autism orgs out there. The one I know of that isn't awful is the Autistic Self Advocacy Network (ASAN), and they have recently had some concerns come to light involving internal racism and bad management. I can tell you to put a country mile between you and Autism Speaks (anything branded with puzzle pieces typically ties back to them). They are the PETA of autism advocacy.