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Looks more like an 18mm to me.
I've just been through a pile of straps and while I have a lot of 18mm that would fit perfectly, they're all extra long straps for my wrist so I have a feeling that they'll be way to long for your gf.
I actually agree with @freddo. That strap looks good. Why not leave it on so you guys can make a decision about a strap together? Its a personal choice...
@Stonehedge I think I need a 17mm strap, any recommendations please?