Because of small feet, I rode a b-nice 154 mtx...no rocker, for a year til snapping the tail, it was a really fun board and imo certianly could be used beyond the beginner level. The 148 should work fine, especially if she is athletic and a bit aggressive.
What Tarz said, and start with board, boots and helmet in the backyard or on a piece of old carpet, teaching her how to roll...lift legs, roll hips, squat over the board then stand, how to tuck and roll (no hands) fall; proper body position, movements and mechanics, show her the board mechanics, how torsional stearing and how to move fore/aft...this will be easier to do when the board is not trying to move on the snow. Have her do it 2-3 times a week before the season,..that way she will be used to the gear, boots can be broken in and/or fitting adjusted to be comfortable. Then on the hill get her some lessons and she just has to figure out how to go, turn and actually stop...and falling will be natural and feel easier on the snow.