cyclocross

The new Boardman MX Race is surely ideal for your other half. It weighs less than 10 kg, even with a suspension fork. The female-specific Hybrid Comp Fi (here) might be worth a look too.

Islabikes make a 700c flat-bar adult model, the Beinn 29, with clearance for 2" MTB tyres and a number of other practical features.

Ridgeback Flight (rigid fork) and Dual Track (susp) models may be worth considering.

As Dom said, the Kinesis bikes are the ones to look at.

The FiveT would be easy enough to run flats on. Just wondering they she would not like drops?

It doesn't mean she has to ride of the drops, she can ride on the hoods, which is just like having bar ends on a flat bar

The women and girls I have seen at local league events have been using standard 'cross bikes. A decent shop should sort her out with something that fits properly, and I'd look at the Islabikes Luath models.

Bear in mind that there are cyclocross race bikes and models described as 'cross bikes but are heavier allrounders (e.g. Genesis Croix de Fer, Specialized Tricross).

A hybrid type with good clearance around the wheels and mixed-surface tyres might be an option if she still prefers flat bars.

Ok, my advice hasn't arrived It was from a female cyclocross rider

all the ladies i know that cyclocross use a 'mans' bike and change the bars/stem/saddle to fit so your best bet is probably to find a bike that's about right and fiddle about a bit

i do know some smaller ladies that have plumped for a kids Islabike 'cross bike as well because the dimensions were just right

IMHO most 'women specific bikes' are a joke, because they tend to be:

a) shorter and more upright while the body proportions of different sexes are not that different, and while an average woman is shorter than an average man, there is nothing to suggest that a ladies' frame should have a shorter top and taller head tubes

b) again from personal experience an average road/cyclocross female rider is waaay fitter and more flexible than an average male rider, so there are more men who would benefit from a 'ladies specific geometry' than there are women. But for some reason you never see blokes ride those.

I am pretty sure most bike shops would swap a saddle free of charge to make a lady customer more comfortable on a new bike though.

Just calling in advice for you.....