Does she want a mountain bike, or something more like a road bike but that will still deal well with light off-road?
How tall is she… tall enough not to need the titchy sizes they make wsd bikes available in?
What does she like and dislike about her current bike, could you upgrade parts instead of getting a whole new bike?
FWIW I got on pretty well with the Scott Contessa series… you should be able to get a 2011 contessa 30 on sale for about £450, but it’s still not particularly lightweight.