if you go for the Saxo 1.1 Desire, like what I have, its very cheap insurance - not boy racer at all, that's the vTS/VTR! The Desire is more "old person" than anything else!! Lovely little motor though, I really like ours, its like driving a tiny go-kart. I think INs group-wise they are 2 or 3, so not the lowest. A Ka is about as low as they go I think, and a Picanto is low too.
If she is a little unsure, then id certainly suggest a more solid small car, so she feels safe, something that's more comfortable so she doesn't get any back pain. The Saxo is very much a girls car, its made for small people! All I will say is, and this may be important to her with the back injury, is the pedals are off set slightly to the left. Means getting in and out is ok (you don't accidentally hit the accelerator, but it does mean when driving your legs are to the left, rather than straight on. Doesn't really make too much difference, but may well do to someone with back injuries - so if you test drive one, take that into account.