Welcome to iMore. If she's a responsible young lady, I would suggest the middle tier of memory (gigabytes). 512gb is a lot and 64gb has one always checking how much memory is left. Nowadays 256gb seems just about right for most. Just my opinion of course.
I saw the lowest possible. Let her experience something similar to what it was like to manage a meager 16GB.
I just noticed that the OP posted in the iPhone 6s forum so the choices are 16-64 and 128GBs