Tell him to install and, when it asks the directory to install, just put it n a different folder...

It might want to instal here: C:\Program Files\Square Enix\FINAL FANTASY XIV
just tell him to add a new path like C:\Program Files\Square Enix\FFXIV

This might fix the problem. For some reason sometimes Vista keeps a underdated list of files so the PC assume the file is there...but when it goes there the file does not exist, causing this issue. And if this is the problem, just installing in a different folder might be enough to fix it.