Again we are talking about people cheating the system collecting disability then working on the side.farmer's market, as long as you don't accept cards including food stamps there is no digital record that the government can subpoena. He could still get reported by whoever hates him, but so are firewood sales.
Total $ amount is also relevant. IRS rarely audit someone actually making $20K for reporting $10K, but if you make $200K and only report $100K, audit is very likely.