The Germans don't really have any more right to it than she does. The most likely story of this bust is from the 5th century until the 18th or 19th centuries, it was buried in the ground somewhere in Italy or former Roman empire.

The person who dug it up and sold it didn't really have an inherent right to it. They just found it.

I say all this not from a legal perspective but from an inheritors of history and culture perspective. We all inherit humanity's history and culture.

While cheesy, Indiana Jones was right: "It belongs in a museum" so it is accessible to humanity. But is that museum inherently this German one?