Mr D having money in the bank account is not income
Actually - let's remember that in Poland, interest in bank accounts is taxed at 19%. So - if your account is earning gross in the UK (which is possibile), yet you're not paying 19% to the Polish taxman, that's yet another black mark.
I think you are making up a lot of stuff as you go around which is sad because your information is generally good...
We're not talking about this though - we're talking about the need to declare the income. Even if Poland has an agreement, in terms of residency, he's tax resident in Poland. His actual tax bill in Poland - well, who knows? That's what professionals are for...It's quite possible that he'll have no bill - equally so, it's possible that he'll be hit with a big bill. Either way, he'll be on clear ground with them instead of trying to operate on the sly.
I'm not sure if one is obliged to inform the polish tax department everytime he transfers money
He'll be obliged to report it at the end of the year, but that's about it.