Oh, well... I would not equate "mathy" (in my mind, perhaps top 1-2% for math ability) with "likely to pioneer breakthroughs in math"! (What would that be...top .000001%?) Gracious. No, she may not be at the level of the top kids on the math team, but she still might achieve great things in a STEM field--especially if she has great writing skills, which you say she does. (I read a lot of scientific papers...)
Also, remember, she does not have to achieve the extraordinary.
Perhaps she could be extraordinary in one area, but will turn out to be more interested in something at which she is less extraordinary.
