By the way, is it a must to pay his social taxes (as delphiandomine stated)? does he need it? or he can simply manage with a private insurance company?
Maybe. I don't know how employment with a foreign-based company is handled in Poland legally. I'm not even sure that Poland recognises employment with an employer that isn't registered at all in Poland.
And what about if the money is sent by western union as millions of people in the world do so? paypal ? online transfering ? cash via post?... then no bank needed, then no taxation needed.
As I said, it's possible. But what happens when the jealous uncle reports him? The Polish taxman isn't known for being weak. In fact, they've forced (wrongly) several businesses out of business, and have refused to follow orders from the Supreme Court.
The other way, of course, is to make sure that he is in Poland for less than the 185 days in any one year - keep the tickets of when he entered and left the country.
Indeed, and keep detailed receipts of spending, along with proof of maintaining a residence outside of Poland.
Really, for the sake of not paying 19% tax, is it worth it?