If you wanna keep her floating I'd go with the helium cga one that looks very similar to the billabong one. And most girls I know who casually wakeboard float on their backs when resting in the water and a comp vest makes that hard for them to do that.
http://heliumwake.com/dyn_prod.php?p=5750VDO&k=126623 And since it's helium that means it looks way cool but is way overpriced.