Quick Google search came up with the actual labor law regarding the discussion with a coworker about how much you make.
Next time one of you has a boss that posts some dumb sign about how you’re going to get fired or reprimanded, print this out and hang it up over the sign.
https://www.nlrb.gov/about-nlrb/rights-we-protect/your-rights/your-rights-to-discuss-wages