For the time being, my take on ZFG is this: Casey saw the rental info card somehow, remembered the name on it and pulled it out of her *ss when put on the spot. From what I can recall, CASEY did not give ZFG's phone # to LE -- she only gave a name and that apartment #..... said SHE tried to call ZFG and could not reach her...... THEN said she didn't know ZFG's number. (Correct me if I'm wrong someone please!)
So I think she pulled the name from her memory, thinking the cops would either (a) not be able to track ZFG down or (b) she would just continue to say "No, that's the wrong ZFG" and they'd believe her.
'Cuz Casey is either on drugs or mentally ill. Or both.