Hi there!

Just as general advice, with regards to your last question:

And is there anything she can do about it now?

We are not a sub for legal advice and I would just generally caution accepting legal advise from anonymous strangers on the internet.

Thank you!

During the Soviet Union's invasion of what was East Prussia and the Eastern parts of Germany in general there was a lot of fear, destruction and violence. There is actually a fairly large literature on the phenomenon you are describing here which is referred to as "Flucht und Vertreibung" in German or "Flight and Expulsion" which looks at the much larger process that saw many ethnic Germans forced out of places like the Sudetenland (the mountainous region of Czechoslovakia that Hitler demanded in the Munich Agreement) and many others who fled fearing Soviet violence.
It's unsurprising, if still sad, that people like your grandmother lost their home, because, as you said, that is war. It's also likely (seeing as how your grandmother was from Prussia) that the place she lived is now a part of Poland as a result of the Potsdam Agreement which moved the borders of several countries including Germany, Poland, and Russia.
As for what can be done about it now, I have to defer to the Mod and say I have no idea, but highly doubt that anything can happen.
There is some interesting literature you might want to check out that does refer to some of these experiences and narratives that were crafted at the end of the war and later such as Fritz Stern's Five Germany's I Have Known, which is a personal and highly readable account of a Jewish Columbia University history professor who emigrated to the United States and his experiences under different German political regimes. His former home is now in Poland. In terms of the Flight and Expulsion literature, I recommend caution. There is some right-wing radical stuff you can find that is pretty revanchist in nature, but if you stick to mainstream, university press-type works, you should be fine. For example you could look at Orderly and Humane: The Expulsion of the Germans After the Second World War by R.M. Douglas.