Perhaps, the same application/s that produced the other files. The resource fork info was possibly stripped as the result of being backed up to a disc that doesn't retain this info or doesn't retain it well enough.
You might want to manually add an extension to this file. Use an extension that worked on the other files. You could also try opening the application/s that were able to open the other files and give them a shot at opening it, as opposed to double clicking on the file.
Quite frankly, if your friend has no idea what that file is and she doesn't consider it a loss, I'd recommend not wasting the time on it. However, if you're set on restoring this file, the fastest and easiest way besides what I've already mentioned, would be to download an app called
File Juicer and give it a shot. If File Juicer can't extract anything from that file, I don't know what can.