I guess it should be better to ask a special Web site that can help you more than me. The problem is quite complex during the Word War II and before.
I noticed your grandmother was born on polish territory.No matter what your grandfather did the child born after will received the polish nationality because of the Right of blood (According to the Polish law, if one of the parent was polish than the child will be polish). If the child is orphan then he will get the polish nationality if he was born on Polish territory (Right of the soil)
The question still remains: Ever your GF change his nationality or the USC (Polish laws) change his nationality because he was married in Poland Territory and remains more than 3 years in Poland for example. I dont know all Polish laws. So you need a lawyer in order to clear your question.