Peugeot 107/Citroen C1 (original models).
Robust, £20 or free car tax (depending on year), nippy enough if she does need to venture out of town, cheap and easy to work on, a hoot to drive, genuine 60+ MPG ability. The Aygo of the same generation is a partial redesign of the body by Toyota to distinguish it from its cousins, but Toyota muffed it up and they're very prone to cabin water leaks which Toyota never managed to cure during the production cycle. Easily as capable as the VW group triplets, rather (actually, a lot) more reliable than the VW group triplets, and you're not putting money in the pockets of a company with low morals like VW.
You could spend a lot more, and get a lot less. I'm presuming she won't want anything too oddball like a Smart or Twizy, in which case there is no better choice of city scoot for your budget.