scared I made a bad purchase for my girlfriend

[Irulan]

There are many variations in body proportion and fit. If the LBS isn't very useful, my suggestion is

1. understand what WSD is and is not. It is most definitely NOT for every woman out there, but for some women, of a very certain body type, it's very beneficial. SO don't buy WSD just because it exists,
2.Try two bikes, same model, different sizes. Example, a 15 and a 17. Try them both, understand the differences and nuances of the fit. Get to feel what too big and too small feel like, so that you can take a stab at "just right".
3. chances are first bike will not be a perfect fit, so just accept it. Getting close is good though. Most mountain bikers don't understand what they need until they've been riding for a while.

FWIW i'm 5'3 and have a 16' mtb a 15' mtb, another 16' mtb and a women's med FST.

But before you do all that, I think you should find someone who knows about fit.

What she said.