*If* she's saying what she remembers, and she believes that someone else was there at the apartment, could she perhaps mean that someone else lived there, but was not on the premises at the time? Like Alcala was staying with a friend/relative, and that was their apartment, but they were at work or something?
If so, then it's probably not during the time he lived in NY...rather, one of the places he visited his relatives or was "on vacation"...which would narrow a timeline down significantly. 1978, maybe?
Best-
Herding Cats
Yes, and wouldn't it have been nice of her to mention what year she was 19 or 20 in? That would have helped.