I would think that any pedal usable with a recessed cleat would be easiest to convince a hesitant person to use. The shoes are more walking friendly and they usually have grippy rubber bottoms so its harder to slip off the pedal if you don't get clipped in right away too.

You might want to consider SPD's for her just because you can get the kind with flat pedals on one side and spd's on the other (or a flat pedal adapter that clips into any standard spd pedal). If she gets nervy about clipless she can always switch to the flat pedals at any time.