She's saying these things to him to reinforce it into his subconscious. One day he may start believing those lies, and one day they may actually come true. Except the tall thing, he's fucked.
No gf saying it of course, but my daily affirmations for the last 20 years are: "you're a piece of shit. You're worthless. You're single because you're worthless. Don't seek a gf, no one deserves to suffer your existence. Blue collar shit work is the only thing you're capable of. You have decayed dog shit where your memory should be. Get fucked asshole, I hate you."
It's nearly impossible to stop because I honestly not only believe it, but it's completely true. It's not something that can be denied. It's a fun life.
I can only speak to the lathe operators, but the pay is not good. Blue collar work is usually underpaid, no advancement, and society generally looks down on you for being "uneducated" and not working a job of value like white collar work.