I think the best way is to teach her to fret an Amajor with three fingers, but with both '1st on D, 2nd on G, 3rd on B' and 2nd on D, 3rd on G, 4th on B' as moving around those open positions around the nut, different versions of the chord work a little easier depending on the chord you're either moving from or moving to.

The same goes for a lot of those supposedly 'easy' open chords like Emajor. Fretting with 1st, 2nd and 3rd fingers makes a lot sense playing things like Edom7, Esus &c, but its a bit clumsy when leading up to various barre chords &c.

It's one of those things where she won't thank you for it right now, but in the long run, she'll be glad she didn't get stuck with a 'definitive' but limiting way.