As others have said or hinted at, it's definitely worthwhile to actually go in and see a good boot fitter. I know buying online is tempting, but boots are probably the most important piece of gear you'll buy, and she needs to see a proper boot fitter to find out what suits her foot shape, what size she really is, etc. Here is a list of fitters on Epic: www.epicski.com/wiki/boot-fitters-on-epicski
Perhaps one of them is near a resort you're going to. If not, here is a link to a listing of shops with certified master boot fitters: www.bootfitters.com/FIND_SHOP.htm
. Hopefully you can find someone you guys can work with.
I wear a women's size 10 shoes, and my boot size is 25. My first time or two skiing was in rented boots, and the on hill rental shop put me in 27s. Definitely not my size, despite what the shoe size/boot size chart says. If I had bought online (which I considered when I was totally new to skiing, before finding my way to the forums) I would've put myself in the wrong size boot, and likely the wrong shape of boot, and skied them out of the box without any tweaks I'd need for proper alignment.