Ummm ... The sizing chart really doesn't help, since her boot is a 5.5. According to the Union Sizing chart either a small or medium binding would fit her boots.
I've found almost all binding manufacturers sizing charts to be a complete waste of time for those who have boot sizes on the border of the binding sizes. They don't take into account the actual footprint of a boot, which can differ considerably from boot to boot. I'm not sure what a practical solution to this may be, but is certainly not consumer-friendly.