There are some non-Schengen gates at DUS that are at the end of the "A" and "B" piers. These can only be reached by entering Germany (Schengen zone): https://www.dus.com/en/flights/transferring

Your girlfriend would have required a Schengen visa to transfer from C non-Schengen to A or B non-Schengen via A and/or B Schengen (not a transit visa, which is what some unfortunate people require to simply transit, even if they remain in the non-Schengen zone at all times).