It's a shame she already knew. I successfully shielded my children from all pop culture spoilers - not just in Star Wars, but films like the original Planet of the Apes, The Usual Suspects, The Sixth Sense, Les Diaboliques, etc. Plus all the Harry Potter spoilers and The Hobbit/The Lord of the Rings spoilers (in the case of both, I read them the novels first). I have strong views about children discovering these stories without crass and idiotic internet memes ruining it for them (and the like).

As for Star Wars specifically, I still think watching them in order of release is better, even if you know the twists. I grew up with the (far superior) original trilogy and wasn't remotely confused. The episode numbers were simply a cinematic homage to old Flash Gordon serials and the like, and starting with "IV" was part of the effect. At that point, no one expected a prequel trilogy.