Have her download the trial off the website. And from there, you can buy keys for every expansion, one by one with the exception of WotG if I recall. Buying the expansions as singles might be more expensive than buying another disk, and is definately more pricey than the $11 thing from Steam atm...but if she's just starting out, she won't really need anything more than core FFXI and Zilart, to be honest.
Edit: Direct link is
http://www.finalfantasyxi.com/
If you don't trust the direct link, go to www.playonline.com and on the top banner it should say something about the vana collection, or free trial.