Getting it unlocked may be problematic as, like here in the US, there are no prepaid iPhone plans in New Zealand (and you're locked in for 2 years if you go contract). With the cost of global roaming, wow, yes, an arm, a leg and a couple of other appendages! If it were for a month or two it would be different but a year of 50c text messages and $1.99 a minute phone calls will be expensive.
In all honesty, her best bet may be to leave her iPhone behind (or just use it as a glorified iPod for the year) and get a cheap prepaid phone over there. Even with that, calling internationally will not be cheap. Since they treat mobile/land lines differently, you pay higher rates to call a cellphone. I usually have my mother call me from her land line, rather than me call her cell. Text messaging, I believe is only 20c per for international for casual use though there may be plans that allow for discounted texting (I haven't checked since Mum & I usually just iMessage).
I hope she enjoys herself. It's a beautiful country.
