I don’t want to spread any misinformation, but if his grandmother knew how to make south Slavic dishes it’s even possible they moved from Bosnia. After AH empire occupied Bosnia in 1878 they settled some Poles, Czechs, Italians etc. there. My maternal grandmother grew up with Galcijans in her village in Bosnia, as they called them, apparently they were Poles from Galicia. After the WWII most of those Poles in Bosnia were moved to now western Poland, most of them to Bolesławiec.
yeah unrelated to this above, their denomination could help a lot.