If she wants to study language i guess it make no difference, but if she wants have social life, may be New Zealand is not suitable. New Zealand is a place with lifestyle. You can have a cup of afternoon tea by beach side and relax.
If you are learning US English, you will find British English accent a bit hard to handle, NZ belongs to the British English stream, so i find US English a bit hard to cope, i can understand it no problem, but the accent will sound funny to me.
I live in New Zealand, got any problem you are pretty welcome to email me or ask me