I travelled with two more friends to Napoli and stayed at Mirko's place:
The apartment is as described.
Both Mirko and his father (Don Antonio) were great arranging our stay and very flexible with check-in and check-out times. The apartment is on the 9th floor and has a spacious balcony, which we could not enjoy as much as we would like to because of the cold weather (still nice, though). We had two heaters at our disposal, so we were very comfortable during our stay. Finally, the location was within walking distance from the city center, so we just walked everywhere. It felt safe overall.