Friedman was a free market sycophant who argued from an ideological standpoint in his latter career (example: here) instead of an empirical.
His Stance: "There is no way in which a union can simultaneously increase the number of jobs and the pay! The law of demand in economics is inescapable." (Quote)
This wouldn't even hold in a modern intro to Econ class. You absolutely can increase the number of jobs and the pay. Friedman falls into the "ceteris paribus" trap all the liberal economists in the 50s and 60s did, and forgets the hundreds of other factors outside supply and demand, boiling things down to idiomatic, ceteris paribus situations.
Industry expansion ALONE affords the possibility for there to be MORE of job X with higher pay, and this is before we consider the simple situation where:
Company X has 20 workers. They ignore safety procedures, breaks, etc. in favor of pushing their current workforce into higher productivity (Y output/hr worked) They COULD hire more people to reach their desired output and while cutting productivity modestly, but they won't; because profit. 20 workers join a union. Union shows up and says, you're safety procedures are lacking and either we shut you down, or you institute better safety policy and working conditions. Company X is forced to agree, because the threat of shutdown < the slightly hire employment costs. Company X institutes safety procedures, and hires N more full time workers to offset the decreased productivity from doing things slower, but more safely. Their profit margin shrinks and life moves on, as long as the firm retains a profit margin, it's incentivized to continue operating.
This is literally the function of Unions.
Supply and demand of labor is a factor, and ceteris paribus, he is right, but reality isn't a ceteris paribus situation. We have empirical evidence of the thing he says is impossible happening.