I agree; It's not worth paying that much to fix an old phone. If you were in the US, the broken phone might still have decent trade-in value, as down payment for a newer model, but that's a Faustian bargain even if available in NZ(?)
Don't worry too much that she/you don't like the 6; after a month you would not be able to switch back
. The problem is it's quite expensive AND Apple recently raised prices in international markets (a bit ludicrous for a 6 month old phone... 'Cos of the strong USD? Oh please). There's a recent rumor that
a new 4" model is coming in the fall so it might be worth waiting to see if that's something you want and if Apple drops the prices back down while they're at it.
In the meanwhile, how much damage did the iPhone take? Is it mostly cosmetic ('merely 'cracks across the screen) or is the underlying touchscreen and other components also broken. If the phone is still mostly functional, I'd suggest buying her an Apple Watch. The entry level is about the same price as a second hand iPhone 5s, it won't be superceded by a new model in 6 months *fingers crossed* and some
early previews suggest it will greatly reduce the use of the paired iPhone so she won't be seeing that cracked screen very often. If it makes it easier justifying getting one for yourself too that's a bonus