You're too good... you're answering your own questions.
Yes, the option key allows you to choose between each OS... which is good becuase he could have opted to wipe off OS X entirely, but that's fixable as well.
Anyway, if she doesn't want the Vista on her mac, you can run the bootcamp assistant wizard from OS X and it will remove the extra partition so that only OS X is left.
You can find that in Finder under Applications, Utilities