Why should she consult you about her decision? Just interested, would you ask her permission before moving away (not necessarily to NZ, even to an area of the UK a day's journey away)?
It does seem like you think she should have asked your permission to move almost. I imagine her "the UK is my life" spiel was while she was considering emigrating but hadn't made a firm decision and didn't want to discuss it until she had.
Skype - obviously no substitute for sitting on the same sofa, but it is free and allows face to face interaction. I probably talk to my father more now than when he was a phone call away.
Re: getting in - the balance of your family (for family stream visas) has to be in NZ or equally divided. So one child in each country and she'd qualify. Grandchildren aren't taken into account if there are living adult children. There's approximately a year waiting time from application for family stream visa to being allocated a visa officer and then about 6 months for processing so (unless she's going to pull some stunts with cycling temporary visas) she has at least a year in the UK before she gets residency.
Does it matter her reasons for going? The fact is, it's her life. You and your husband now have a choice - you can be bitter, upset and hurt, let her know it and not make the effort to keep any form of relationship going.
Or you can be upset and hurt - but tell her you love her, will make the effort wherever in the world she's moving to and keep a very good relationship alive with Skype, letters, photos, visits.
When we moved here (2 years ago) of course my family were upset. But the life we can offer our children here is orders of magnitude better than the one we could offer in the UK. My family all chose the latter option (thankfully). They've been out to visit each year, we're going back next year for a holiday.
My MiL arrives in October (also emigrating), can't wait :)