Two ways.

1) use a USB card reader. Insert the mates card, open the card up in my computer, and highlight everything in there, copy it, and paste it into a new folder on your desktop. Take his card out, put yours in, and then copy everything in the new folder onto the new card.

WORD OF WARNING. To do this you need to have SHOW HIDDEN FILES AND FOLDERS ticked in the Folder Options (top of the window in any folder: TOOLS>FOLDER OPTIONS>VIEW, scroll till you see that entry)

2) use the supplied TomTom Home software. Put your mates card in your TomTom, and connect the TomTom to the PC using the USB cable. In the software is the option to MAKE BACKUP. Do this. Then, disconnect your TomTom, swap his card for your new one, and reconnect. In the same software is the option to RESTORE BACKUP. Do this. If it says anything about it not matching, just click ignore or whatever it is, so that is restores the back up to your new card. Disconnect after it's done and it reboot the tomtom. Should be fine after that.