Does she have a virus?
Definitely not... at least, this is not a symptom of any kind of malware. Modern malware does not go deleting files and folders as it did back in the early days of malware. These days, it's business... a way of making money. Malware wants to hide and not draw attention to itself.
Folders should not just disappear. Is it possible to "hide" a folder?
It's possible to hide a folder, through a Unix command executed in the Terminal or through some third-party software. It really couldn't be hidden accidentally... it would have to be a purposeful thing.
It's far more likely that she accidentally deleted the folder. It's just a matter of pressing command-delete in the Finder with that folder selected to move it to the trash, and from there, most people will empty their trash from time to time without checking to see what's in it.
You can search for the folder, or a file that should be in that folder, using baltwo's recommendation, or you could download and use EasyFind. (Note that, if she's using Mountain Lion, it will block EasyFind as it is not "code-signed" to identify it as being created by a known developer. It's safe, downloaded from the link I provided, though, so you can open it on Mountain Lion by control-clicking the EasyFind app, choosing Open, and confirming that you want to open it when asked.)
Unfortunately, if the folder is nowhere to be found, and she's not backing up her computer, it may be completely gone. In which case, your only option would be to use recovery software to try to "undelete" those files. That may or may not have any success. See: