Pictures won't show on camera

Sister had uploaded pics from Nikon SD card to PC, then made
the mistake of erasing many files on SD card instead of on PC's
copy.

Did that for her, so camera card now has all the original pics.
BUT, the camera doesn't see the pics downloaded onto the card
from PC - only the few pics that had been left on the card.

The PC can see all the pics on the card, as can the photo
printer.

Anyone know how to make the camera see all the pics?
(I believe Nikon has some hidden directory on the card that
controls which pics it sees.)

Is there a way to make the camera see all the pics that are on
the card?

Did you read any information about memory cards? Most likely not or you wouldn't be asking this.
Memory cards and cameras are not hard drives and computers. Cameras only read the files that they took prior to any changes such as uploading. The files are there but the camera can't see them.

Now this kind of activity is about the easiest way to make a memory card fail. If you'd read your information you'd know this already. So don't cry when your card fails.

Move the photos to the same location as the other photos on the SD card.