Snowboarding was developed in the U.S.A. in the 1960s and the 1970s. This sport requires lots of movements and it takes out lot of energy. If your girl just started this sport, then it's very important having special clothes. You have to keep your legs in warm and be free in movements.
Having special clothes is very important, that's why choosing girl's snowboard is quite difficult:
1) make sure pants are easy for moving;
3) breathable material.
All that is important. Make sure pants are made of water proof material and they're light in weight. Regarding colors, choose the appropriate to your girl's snowboard jacket.