Like your gf my natural squatting stance is 17" but I ride with a width of 18". I started my binding width at 17" but it just felt too narrow in practice. 18" felt a little wide when standing still but perfect once I started moving.
For what it's worth my shoulders are also 15" wide. The guide you found is probably not relevant for women since men tend to have broader shoulders relative to their hips. I'm 5'3" with short legs, and if 17" is too narrow for me I'd be surprised if it worked for someone who was 5'8". But it doesn't cost anything to experiment, so just play around and see how you go.