In order of release date: IV - V - VI - I - II - III. There are a lot of references in the movies filmed later that are meant to give you chills because you know how things turn out later.
On a side note, I've always found this review spot on and very funny. In case you are in the mood to hear someone criticize the new movies for... about as long as the movie itself.
IV. Then stop there for a while. Star Wars has been out for what, 35 years or so now, so if she hasn't seen ANY of them yet, they may simply not be her cuppa tea. If she likes it, go with V and VI, and then if she still likes it, stop there. No point in ruining her enjoyment by showing her I-III.
Start with the old ones, because that's what most of the cultural reference comes from. Then, when she's watching some movie and they're talking about the Death Star or wookies or whatnot she'll know what they're talking about.
