The entry and residence conditions and the list of countries whose citizens have the right to enter and stay in Poland under the visa-free regime can be found on the website https://udsc.gov.pl/en/cudzoziemcy/obywatele-panstw-trzecich/chce-przyje...
If your employee entered Poland under the visa-free regime, he/she may work in Poland provided he/she has a valid work permit or a valid declaration of entrusting work to a foreigner from a PUP.
Remember that visa-free travel entitles one to stay in Poland for 90 days within 180 days. If a foreigner intends to stay in Poland for a longer-term, they should apply for a temporary residence and work permit with the Province Governor competent for the place of residence during their legal stay in Poland.
