This always works for me, but maybe it's just because of my own personal sock knitting gauge, but if her shoe size is an 8 1/2, when you are doing the foot part knit 8 1/2 inches from the back of the heel and begin your toe decrease.
I'm a US size 7 shoe. I knit 7 inches from the heel for the foot length then begin the toe decrease.
This has worked for me for everything from a size 5 to size 10 foot for women and men. The only difference with men's socks is that I use more stitches. I'll use 64 for women and 72 for men or multiples thereafter depending on the pattern.