In an ideal situation. In a realworld situation that means running wires through walls, not something done easily in this situation. Hiding a camera in a bookcase is easy. Secretly running wires from that bookcase to a recorder in the next room is not.
They literally kicked his upstairs neighbours out and moved in there themselves. Once you’ve done that, dropping wiring down from your floor to his ceiling become trivial (at least for certain types of building construction).
In an ideal situation. In a realworld situation that means running wires through walls, not something done easily in this situation. Hiding a camera in a bookcase is easy. Secretly running wires from that bookcase to a recorder in the next room is not.