I don't think Chris is committing fraud, but mental disabilities are a lot softer than physical disabilities. There's a lot more room to debate the specifics, and ultimately someone will need to make a judgement call. I'm guessing the SSA got a diagnosis of autism from a professional, had their guys look over it, and since autism can't really be cured, they approved Chris. I don't claim to know that Chris could sustain a full time job, necessarily, but Chris could definitely sustain a part time job.
Chris' autism only affects his social skills. Theory of mind and empathy and things like that. In comparison to lower functioning autistics, Chris doesn't have episodes where he freaks out or anything like that. (Well, I don't know, maybe blue arms induce sensory defensiveness in him?) All Chris needs to do to maintain a job is keep his mouth shut and do his job. He's capable of doing that and Barb would force him to do that when she needs him to.