Yes you can fly in F and request a companion upgrade for someone on a separate itinerary... obviously they just have to be traveling on the same flight as you.

Oh and when we say 'F' it just means the F cabin, not the fare class... you can book in whatever fare is cheapest that gets you up front.

So... book your ticket up front, then hers in coach and call to request a companion upgrade for her. Or, you can book her up front and use some miles to reduce the cost and book your ticket in back and take your chances on the upgrade.