I made considerable progress when I discovered that I couldn't drag and drop the files onto the Flash Drive icon. I had to open the flash drive and drop them "inside".
There is still something wrong. Dragging it to the icon should have worked (again, as seen in the video I posted). My only thought is that the number of files is quite considerable, and Finder is struggling on that older machine. Finder, especially in older releases is not the most solid file manager out there.
I learned what little I know about computers starting with a C:> prompt, and my mindset is keyed to hands-on file management, organized with a logical directory tree and filenames. From that perspective, I can tell you that Apple's file management procedures are simply terrible.
There is a logic to the Mac file system layout that doesn't really vary much from modern Windows, though I can't say for certain if your wife adhered to it. Much like Windows, OS X provides some nice defaults (My Documents/Pictures etc) that users can ignore. I cringe when I see a file in C:\, just as much as I cringe when I see a picture sitting in Documents.
This is why Apple is moving away from exposing file structures to users in iOS and slowly OS X. Only people who wanted to fully learn the computer, no matter the OS, bothered to adhere to any logical file layout. For Apple, their new direction is to just associate files with the program that works with them. Want to edit a word processing document? Open the word processing program, and do any save, open or sharing via the program, and not a file manager.
In general, I appreciate Apple's attempt to rethink computing. There is a lot of legacy cruft every system drags along with it, including OS X. We the power users have gotten used to the cruft and work with it, sometimes even screaming loudly when it's changed. The people who never adopted the cruft and started on something like iOS are doing some amazing things with little to no help. Opening the power of computing to more people is a noble goal.