You are right, they would if it was that easy!
She is confusing the ability to place the cabin there (temporary structure with no foundations), which she may well be able to do and use it as an ancillary building to her main home, with the planning consents required to live in it as a residence on a permanent basis. The first is probably straightforward (although any facilities like toilets etc would then bring in a lot of building regs and suspicions on the nature of the usage), the second she is very unlikely to get on a greenfield site unless she can show a business case for doing so in terms of the scale and nature of the (say agricultural) business she is operating from there. Otherwise it falls in to the new housing in the countryside presumption against that nearly all councils operate (some small exceptions like the LILI scheme in one area in Wales)
She would have to show that her business requires her to be on site 24x7, that there are no suitable existing properties available within a suitable radius, come up with a thorough business plan for the business and potentially then still only get a temporary permission for 5 years or less, following which if the business plan is proved to be rubbish they can require the property to be removed.
The councils policies should all be online in the local plan, worth looking up to see what they say.