1 Xbox 360 formatted USB drive
1 pc/laptop with a USB slot
You will need to remove everything from the USB drive. You can copy it to your Xbox 360 for safe keeping. To be clear, there should be no profiles, demos, videos, tracks, NOTHING on the USB drive.
Then fire up your Xbox and go to:
My Xbox > System Settings > Memory > HDD* > Games > Trials HD
Copy each track that your friend made from your HDD* to the empty USB drive.
You are now finished with your Xbox.
On your PC, search for (and download) USBXTAFGUI_v44, this program allows people to view Xbox file structures on Windows based PCs.
Plug the Xbox USB drive that contains ONLY the tracks that you want to send back to your friend into an available port on your PC. Unplug other USB drives from the PC.
Launch USBXTAFGUI, go to "File" and click "Open First USB Drive". Make sure the "FileSystem" tab is selected and then expand every subsection under the Data Partition,
Click to select the final section like so;
The items on the right, if you ONLY have the Trials HD tracks on your system, will be the Trials HD tracks that your friend lost access to.
Simply select them 1 at a time and drag them to your desktop;
Now you can just send an email to your friend with the tracks as attachments, and he just needs to set up an Xbox USB Drive with a single Trials HD track (and nothing else) and place the tracks you send him on it in the same manner as listed above.
Please note that doing as I have listed is NOT endorsed by Microsoft or Redlynx. You do so at your own risk. Any data loss or other bad stuff (it's a legal term) is solely your own fault.