I’m not seeing why he shouldn’t have discussed and learned the fact that he was being paid a lot less than he realized he could be, apart from hurt feelings. Perhaps there was a good reason for the discrepancy - like pay being based on education level - but again I don’t see why it would be inadvisable to learn how the pay system works.
I don’t see how advising people not to understand the pay system at a given company or how to figure out if there is an unfair pay equity situation happening is a good thing. In this instance if this guy developed a problem with you because of what you said that his mistake; he should have developed a problem with management (and maybe thanked you for opening his eyes).
I did once start as a newbie at a company during a time when they were desperate for workers; my boss (though we did the same job) was hired when times weren’t so bad. I learned after a few months that I was making more than him despite having less experience and being in a subordinate role on paper, but kept that quiet so he wouldn’t get upset… so I can see a bit of the motive to not discuss such things, but in the long run it would have been better to do so. Companies seldom go out of their way to over-pay employees, but spend considerable amounts of time figuring out how to under-pay them. Those employees ought to learn their own value if nothing more than to not be taken advantage of.