Sky normally permits people to keep their set-top boxes when they cease to subscribe. (Indeed, I've never known anyone who's been required to return their box).
A Sky box without a subscription becomes a generic 'free-to-air' satellite receiver, which can actually get MORE stations (in it's 'off the shelf' state) than a Freesat box can. (Freesat provides a subset of all available free-to-air satellite channels, although its possible to add the more obscure ones that are available manually).
Assuming that your neighbour is able to keep her old Sky box, there's very little point in her purchasing a Freesat one. (A Freesat box might offer a better electronic programme guide, or some other type of 'add-on' service, but it won't get any extra channels).
So, simply by keeping her old box, your neighbour should be able to receive all of the channels listed here:
https://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
As Carrot99 has mentioned, Sky offers a 'Freesat from Sky' card (which is NOT the same as 'Freesat') for a one-off fee of £25. However it's actually a waste of money for most viewers, as all it adds on to what you can get without it is access to a a VERY small number of 'free-to-view' channels (which, unlike 'free-to-air' channels, are encrypted). Apart from local TV stations, the only 'free-to-view' channels currently transmitted via the Astra satellites (which Sky uses) are 4Music and Yesterday+1.