I take it you don’t believe the workman is worthy of his hire. It doesn’t matter if the guy is “illegal” or not. If you hire someone and don’t pay them, it’s theft and the person who does this is scum.
What is this, a loophole in the “thou shalt not steal” rule?
The employer hiring the illegal is no less guilty of a crime, and should be prosecuted. But from the perspective of the illegal... karma is a female dog.