Boot an Ubuntu CD (get one from your friend). Select "try Ubuntu without installing". When the desktop comes up, plug in the USB drive to see if anything shows up. If it does, click on it to see if your files are there. When done, do a right click on the desk icon for the drive and select "unmount". After the icon goes away you can unplug the drive.
Maybe he reformatted your external drive with a Linux filesystem that Windows 7 doesn't handle. If that is the case, you'll need to (maybe have him) offload the files again to another drive, reformat, and load them back. Alternatively, buy a new 500 GB drive, use Ubuntu to copy the files to the new one without reformatting the new one (so it stays in a FAT32 format). Do this alternative only once you verify that the problem is the drive is in a Linux format. Then you can reformat the old drive and keep double backups (a good thing).