Social Security is a program that workers pay into. You pay into it and you get something out of it when you get old or disabled. And part of its protections apply to your spouse and your children as well, as long as they become disabled before adulthood.
This is Bob's money that he is absolutely owed.
Tax evasion is dumb to bring up regarding Chris. Technically speaking he's evading some taxes, but it's on par with a waitress not filing taxes on her tips. Hell, it's way less significant than that. Waitresses make a lot more money than Chris makes.
(Or actually, with Trump's tax changes, Chris might not owe any income taxes because he simply doesn't earn enough. The standard deductions probably wipe out all of unreported Chris' income.)
If Chris really wasn't disabled, sure, he should be reported. But I think practically speaking, the moralistic "always report leeches" is a dumb argument in this situation. Chris isn't suddenly going to gain 20 IQ points. The man's brain is broken. Always has been, always will be.