it seems like the drive is error-free based on the scans I've run.
What scans did you run? WD hard drive diags?

the content of the drive (mostly a series of large video files), but I can't seem to open any of the files.
What video player did you use? If it was Windows Media Player, you might not have the proper codecs. Like fink suggested, VLC will play just about any type of video file.
https://www.videolan.org/vlc/

Make sure your friend knows that putting files on an external drive is NOT a backup. You should always have at least 2 copies of your data in different locations. The more copies, the better.