Things happen during war and things are lost, so they need to get over it.
It was lost, but now is found. If truly theirs, then why should they "get over it?" Would you not want your property back, especially if it had some sort of specific sentimental value? Of course you would. Would you try to get it back if there was a nice legal means to do so? Sure enough. I don't understand why some folks are faulting Poland for trying.
It very well may turn out that the collector in this case is the unwitting victim of being in possession of property for which he actually had no right to own.