There are still many UK citizens living in Germany, who do not have yet an "Aufenthaltsdokument-GB". If you know someone who lives in Europe, let them know it's not too late to get the required legal document, if they don't have it yet.
They should therefore approach the foreigners authority (Ausländerbehörde) responsible for their area of residence in Germany and apply for the required document (Aufenthaltsdokument-GB).
Otherwise, they could be considered a short-term visitor and ...