Hmm, I had a thought, that it might be from the direction of Hamburg because of the words "flüsterst" and "durch (mein Fenster)" at the beginning. He pronounces both with an ö or o (flösterst, dorch), if you understand. I know that from some people from that part of Germany. But that was just my theory, if your friend is sure, then it's mostlikely Saxon.
As for the GDR theory, I'm not entirely sure. The YouTuber wrote to me that it must have been a site for free music for videos. In his Minecraft Bill Gates video, he linked ccmixter.org below for his music, but I didn't find anything there. But the artist he linked to also has German songs, some of which are similar, I would say? But he mentioned that the site probably doesn't exist anymore in his response last night.
But so far, I've listened to a few songs with titles like "Nachts" or similar from the genre and from the GDR era, and it would fit very well, it's really difficult. Maybe the YouTuber also just doesn't remember where it's from and assumed it must be from the internet.
"