Stiffness has little to do with how high or low end the boot is, it's a function of application.
She'll almost definitely need a men's boot.
And the advice is always the same: Go to an actual shop with a big selection and try on as many as you can. Bring a book, keep each pair on for at least 20-30 minutes while you walk around the store or browse the merchandise - but stay on your feet.
Thank the store for their patience and help by actually buying the boot that is the most comfortable from them.
"Never attribute to malice that which can be adequately explained by stupidity."
10/11 - 24, Smote on Feb 13