100% approved by our army of gals and our field report earns us a few bucks.
Two could be better than one. It wouldn't be a bad idea to give her one bottle to use at home and another one for work. Or maybe one bottle to use when exercising and one for long car rides.
Pick the right size. Whatever sized water bottle she uses now, you should give her a jeweled bottle of roughly the same size. She will enjoy her gift more and likely appreciate that you know her tastes so well.