If you buy a men's / unisex geometry bike, bear in mind you will almost certainly need to change the saddle and possibly the bars (to a narrower one and thus the tape) and also fit a shorter stem or at least flip it. This lot could cost you £100. Something like a
Ribble, Dolan or Planet X build would enable you to do this without cost
Bike fit is key - regardless of WSD or not
I have one WSD bike and one unisex bike - however the latter was built for me so bars, stem and cranks fit me
In terms of suggestions, that budget should get you a top alu/carbon forked frame with 105 gearing - I'd let her choose one that pleases the eye. At 5'7, there should be no issue with finding a frame, be it men's or women's - I am somewhere between 5'9 and 5'10 and ride 545mm frame. I am thinking she'll be looking at a 53cm.
I'd go to a big LBS that has a range of brands. All of the big companies do WSD - Giant have the Avail, Specialized the Dolce, Trek the Lexa, Cannondale the Synapse feminine, Bianchi Dama etc