Cyprus is probably easier and cheaper - everything in English and the same tax advantages as for Delaware with a few extra.
Isn't the Cyprus benefit (9.5% as opposed to 19% tax on dividends) finishing at the end of the year?
The real issue I can see here is residency - unless the business is properly registered in Poland, he's not going to be able to use it to get the residence permit.
Dividends would be taxed at 19 percent, salary at 32 and interest in a variety of ways - that's just the polish tax.
Is there anything stopping him from paying himself dividends every month?