I'm not too sure other conditions are like, besides your friend's living in a top-floor of apartment in Copenhagen, Denmark - should be quite cold(or getting) there now, correct? Did she - leave door or window open? Is there any place/gaps etc in her apartment where cat could somehow sneak out/in, hide or escape? If there's no "secret path" to outside, the possibility is that her kitty may be hiding somewhere inside of apartment where secluded & warm.
This happened to me when I first moved into my current apartment:
I was in the room with absolutely no door or windows open at all, and noticed Ku Ku disappeared! I was really freaking out as I had no idea where she went.
There may be some tiny gap or something I didn't know that she may be able to squeez herself in & escaped? I called her name, looked around everywhere but I couldn't find her. Then my landlady showed up, she helped me looking around in the apartment.
Guess where I found Ku Ku? She was, hiding by a corner in a small space under a living room floor! My apartment is detached "cottage type" studio. Living room floor is raised approx. 1/2 feet from kitchen where a couple of staircase connects these rooms. There's this open space below living room floor where I didn't even really know it existed until then(thought it was tiny space, NOT a size of an entire living room floor). Ku Ku was sphynxing at the far end corner of this space, listening to her meowmy going frantic...
I felt really relieved to see her inside of the apartment...seriously thought that she escaped, y'know? Needless to mention that after this incident, I attached a board & sealed the space so she can not get in there...
Hope your friend will find her kitty