here in Israel before moving to Poland?yes
, they should have all the proper info regarding moving back to Poland. It is called relocation. She is transporting < mienie przesiedlencze" and from that Polish citizens are exempt from paying duty upon crossing the border.
She has to prove that she was living in Israel by showing her paying stubs for the last 12 months (provided she was working) and/ or copies of the bills for rent, electricity, phone bill for the last 12 months, OR/AND an official letter from the consulate that she was a resident of Israel for the last 12 months. I moved over a year back from Canada, had to show the bills. The more she has, the easier it would be getting her stuff clear. Trust me on that. I had some problems in a process, since I was studying in Canada and discarded all the bills. Managed to prove that I was a resident nevertheless.
She should check all the info I have provided with the Polish consulate just to make sure.
One only pays duty on new /unused things, so make sure that even if you buy something new, it looks used:) unpacked from the original package- you don't want to be paying duty on newly purchased stuff. I understand that upon moving people purchase some new stuff to bring into Poland at times, so they duty officers are very diligent when it comes to checking that:)
Good luck in moving:)