If I thought he was serious, I damn well would rat him out - and I'd tell him I was ratting him out, too.
But I don't have a legal duty to do so.
One of the fundamental weaknesses of the law is its arrogance. Its actors now think and act as though the law alone can hold a civil society together. The belief is that "the law" holds that much power. It doesn't claim that it IS the only power holding society together, and it always welcomes outside help, but its claim to superiority over all else does not serve it well.
“Our Constitution was made for a moral and religious people. It is wholly inadequate to the government of any other.” ~ John Adams (Oct 11, 1798)