Without actually seeing her do the skill, it sounds like she could be arching on the way over the top of the bar instead of coming over hollow, with her toes first. Learning how to swing on bars, especially swinging giants, can be extremely scary, but they will eventually become fun. Fear is a constant factor in gymnastics, and over coming fear is something that gradually becomes stronger.
When you do a giant, you there are 2 basic shapes: A scoop (hollow) and a whip (tight, slight arch). As you begin swinging down for the giant you want to be hollow, then right under the bar you whip, then you tap (kick) your toes back up into the hollow shape. Many gymnasts do not keep kicking their toes over the bar, so they end up going around arched; which can also lead to not completing the skill. Here's a video a girl who is not quite hollow going over the top:
http://www.youtube.com/watch?v=U7OQhNajuho In this case, she needs to whip a little later and kick her toes up and over the bar harder after the whip.
Hopefully that'll help you out.
Also, as far fear, I suggest doing lots of reps with spot, and trying to do more than just one in a set.