1) copy off the files, burn a CD for your girl friend, stick the card
back in your camera and format the card for your camera.
2) when you finish with her card take off your images, and she should
format the card in her camera.

Cameras are image making devices, not hard drives. In a way jpegs are
jpegs, but each camera has its own way of making the jpegs, and
recognizing jpegs. If you switch cards between D50s that shouldn't be
much of a problem, other than a numbering issue. But between different
models of cameras, one camera doesn't recognize the others files
because it is looking for a specific file structure. It is always good
to format cards in the camera they are in if you have switched cards
between two different cameras.
After I have down loaded my pics to the computer I always reformat a
card. Though it totally destroy data reformating gives access to the
whole card.

Tom