A brace might add to stability (which she doesn't need, at this point), but wouldn't protect her from a bump on the knee. More likely, it would inhibit her speed and movement. The best protection against a bump is a bumper (i.e., lightweight padding), which she already wears, inside the sleeve, against the outer side of her knee. That padding didn't protect her from the bump she got, but I imagine she's already decided where the optimum placement of the padding should be, to allow her the most flexibility. Even the best protected hockey goalie or baseball catcher or home plate umpire can occasionally get pinged in the tiniest vulnerable opening of the protective gear.