Stop being ignorant. I took foundation Maths twice and got a D, then I took higher and got a B. Why does it bother you that she wants to develop her mathematical skills and enter a science route? Do you think she isn't capable because of some mere GCSEs? (which aren't the best predictor of a-level success).
I personally think your a bit full of yourself based on your GCSEs. Yes foundation is easier but I made a transition from foundation to higher and it only required me learning a few more topics. There is always room for improvement, people develop at different rates. I think you need to stop being so blind and arrogant and realise that she is ready to change.
@ the last line, you're a joke mate. I got a B in GCSE yet got 4As at AS-level in Maths, Fmaths, Econ and History, whilst somebody who got 8A* at GCSE got ABBB.