Bullshit I remember my friends dad up in Chicago being a non union dry wall guy, he was middle class with benefits, those non skilled non union jobs got flooded and undercut by illegals dropping the pay rate to around $10 bucks an hour today.
A dry wall guy isn't a floor sweeper.
So a job that requires little skill got undercut by people working for less. (Not all of whom I suspect were illegals, but never mind.) I suspect your dad's friend wasn't getting anywhere near the union wage, so he was undercutting the union guys before the "illegals' undercut him.
The thing was, most of those construction jobs were union, until people figured out why would you want to pay a union guy a premium wage when the non-union guy will work a lot harder?