Why would you assume that what he is being paid is his current market value? It's entirely possible that he is being paid well under his market value, thus creating feelings of being underpaid.

People are often underpaid compared to market rate because jobs are not a perfectly liquid market. This is due to a number of factors: family situation, workplace social bonds, confidence in one's abilities, fear of change. It is very naive to believe that the labor markets are efficient. This is why some companies are able to take advantage of employees and get away with underpaying them.

"The market has said that you're currently worth/getting paid X"

Has it? Seriously, who is this all powerful "market" that one can go ask and get an exact answer? What does "market" have to say about employers simply giving anemic raises, if they give any at all, to employees they believe will have trouble finding new employment?

Someone's current wage is not correlated with the market wages for their current position.

I'd agree with your premise if each side in a negotiation had perfect information, there was no power asymmetry between parties and humans were perfectly rational actors 100% of the time.

I see what people in other places earn. Last time I mentioned I was in Spain a number of people said that the salaries sounded low. Still the quality of life is decent here or I would move.

Its often the case that nurses and teachers are considered underpaid as well. While bankers are overpaid. Would you agree with that?