The best part about investigations like this is they go back years....so the guy has looong forgotten about this sale or that sale, but they all become evidence he'll have to explain...
Plus, if he didn't report the $300K as income, it's a tax issue too.
People always say "the government has a million other things to do, they'll never pay attentio to me and my little scam!" And that's true - UNTIL the government decides to focus on you, at which point they give ALL their attention.