If she's not already an LPN or RN then it would be faster and easier to get married, apply for US greencard and then start school. There's lost of hoops to jump through and these days you really have to prove that you're marrying for love and not for a green card. Processing time is probably over 12 months for a marriage greencard.
As a Canadian LPN she'd apply to the state BON to get her credentials assessed and probably meet state requirements since her education would be equivalent to ADN.
Aww to be young and in love with an American....been there, done that and I'm back in Canada.