Usually the rule of thumb is 1 1/2 sizes down, but my Fox Forma Pro boots are 2 sizes down (I wear a women's 10 and the Fox 8 fit really well.) I'm not aware of any women's specific boots - as far as I know if the adult mens versions don't go small enough, women with small feet wind up wearing kids boots. Usually the biggest problem with women wearing mens boots is that women's calves can be bigger, but I found that the models that accomodate knee braces or shin guards left plenty of room for both my calves and shin guards. Best bet is for her to go and try a bunch on and see what works.