Skipping classes, if he was resident in Poland he would not need to do a tax return in the Netherlands which he claims to do. And he claims that the Dutch tax authority is perfectly happy with his returns and not paying taxes. So since he does tax returns in Holland he is likely seen as resident in the Netherlands.
As such he will eventually get a nice tax bill from the Dutch tax authorities.
He is also completely unaware that years in EU do not count towards state pension in NL. In fact quite the opposite, you can volunteer to pay the extra social taxes for AOW so that you do build full rights to state pension in NL. If not the other EU country is expected to cover state pension for those years at their local rate according to that countries legislation.
I would take his "knowledge" with a grain of salt!