If I get her a new bike, top of the list is the new Charge Bikes Hob. It looks nice (and looks even better in the flesh), it's got a step-through frame so she can wear a skirt, and one gear is more than enough for riding around London.
On paper £630 looks like quite a lot for such a simple bike, but the frame's good (Tange tubing - not some unbranded old crap) and all the components are top quality, so it's actually pretty good value.
NB. My opinion of value when it comes to bicycles is totally skewed and is probably best ignored.
Next on my list is the Creme Cafe Racer. It basically does the same thing as the Hob, except with gears (7, in the hub), dynamo lights, a rack a slightly more euro feel. At £850 though it's definitely at the luxury end of the spectrum — probably a good choice if you can get cycle2work though.
I can't help thinking though, once the strikes are over, one of Boris's Barclays bike might be a better idea for many commuters than actually owning a bike. A bit like cloud computing I guess — why store stuff on your own computer when you can store it in the cloud? And why own a bike when you can just use one as and when you need it?