When it comes to gun fitting a surprise is probably not the best idea, this is something she will use for years, or not , and how it fits may be the thing that tells how long she will even bother.
Better to find people in the area that will permit her trying different guns, or fess up and take her along for the fitting.Even a 20 ga that is poor fitting can be painful an quickly turn off shooters. We need all we can get and if your wife / girlfriend shoots,odds are the rest of your family will join in without question.