Timeline doesn't make sense, she gave him duplicate key 3 years ago, and she still lives on her own in that apartment, that means she was living there in high school. What about parents?
@yadua that hasn't been such an uncommon thing in Japan for ~3 decades now...some parents allow their more mature children to live in apartments closer to school, depending on the circumstances. Japan is quite safe for late-teens to be self-reliant, although I'm not sure about the cost of the apartment...