Uninstalling the app and re-installing the app from the store did not work.
Running through logs, temporary file directories, default file and program associations, I kept coming across various "FF" entries that struck me as odd or out of place. After a week or more of troubleshooting to no avail, the user sheepishly confessed they had previously installed a free file opener called FF something (I think it was called Free File Opener?) and then uninstalled it before contacting me (why do users always leave out the most important information!?).
It basically hijacks hundreds of file associations on the computer. The only way to fix it, was to hunt down the FF app, re-install it, use it to reset everything back to default, and then uninstall it. After doing this, the attachments started opening properly.