For what I know is if you get a job offer from your employer, you need to work with them only.
You're absolutely right, generally speaking. And there isn't any open work visa you get, following on from getting a job offer. See the information given on these links to the INZ website.
http://www.immigration.govt.nz/migra...haschanged.htm http://www.immigration.govt.nz/migra...anotherjob.htm
There are only a few kinds of open work visa, granted in particular circumstances (so your friend would have to fit in to one of these categories to have an open visa). There's full Residence, gained by applying under the Skilled Migrant Scheme.
http://www.immigration.govt.nz/migra...nt/default.htm There are the Working Holiday Visas, which you know about. There is the partner-sponsored work visa, where someone's partner is a NZ citizen, or has Residence or a work visa of their own.
http://www.immigration.govt.nz/migra...milystream.htm Then there is the SMC Job Search Visa, and the Silver Fern Job Search Visa, which both allow someone to work anywhere, but ONLY for nine months WHILE they are looking for a skilled job offer, and if they don't get one, they have to leave the country.