Also.... bumpy may be on to something. If they are Nikon raw files, they will have an extension .nef, and you need a raw file converter software. But your friend should still be able to see the previews in the camera screen. And, when you put the card in the reader and just open it like a folder, there should be an icon for each file.
And not to be insulting.... but is he absolutely sure this is the card he used to take the pictures? They all look the same unless you number them.
If he can't see the pics on the camera LCD, he needs to take the camera AND the card to an authorized service center. He should try a different card in the camera first.
ALWAYS format a card IN THE CAMERA before using it.
ALWAYS BACK UP YOUR PHOTOS DAILY BY COPYING THEM TO ANOTHER DEVICE OR PREFERABLY TWO OTHER DEVICES. ALSO LEAVE THEM ON THE CARD AS THE 3RD LOCATION. GET MORE CARDS IF NECESSARY.