I wasn't clear I guess.
interprocess communication on vista is locked down like no other; (and this is exactly what loading additional DLLs is doing - literally inserting code into FFXI's application space during run time) try ticking the little box if you haven't yet; it's not like it can hurt you.
for another thing, you have to be on an admin account for disabling UAC to actually approximate admin level access; a general user is *still* going to be on user level access. (being that it's independent of UAC).
I have run and successfully loaded a wide variety of plugins in vista x64 *with* UAC enabled; so I promise you it's something your friend has configured wrong. (there is a remote possibility that you have to be on an admin -account- but I find that unlikely)