First, the concrete finishing contractor hired the man in question. I didn't hire the concrete finishing contractor; my sub, the framing contractor, hired the concrete finishing contractor. That's a hireee of a sub of a sub of an owner builder constructing his first house. I didn't hire the guy nor did I even know he would be there. I should have, but that's now how I ran that job because I didn't know any better at the time. So sue me.
Second, I don't know for a fact that he was an illegal; nor did I have initial reason to expect that he was an illegal, in part, because he DID understand English. It is in retrospect that I have come to have visceral reason to suspect it, in part because of the way the sub was treating him.
Third, although an injury could have happened, most people who trowel concrete slabs don't risk their lives doing it. He was only on the job for one day. This was a slab poured near the end of the framing contract, not the foundation. The sub-contract was under the framing contractor who had proceeded over the rest of the job without indcident, and he was running under union rules (for insurance and health care reasons).
Finally, this was 1991. At that time, I was only dimly aware of the infiltration of illegals into the construction trades, in part because its scope was far less obvious than it is now and the rest was because I was an owner-builder constructing my first home in Northern California.
I might advise you to read a little more carefully and ask a few questions about the circumstances to advise me a little more effectively (much less politely), but then, I already know that's a lost cause.
They you can't be held responsible.
In big projects like this, it's very hard to control who is who and I understand that, but if we don't do all we can, then the situation (illegal immigration) will never go away.