I have encountered a few athletes who do that (similar to 'dragging' one hand in round offs); if one hand is getting left behind it means she's not popping from it, so have her do lots of handstand pops (easier variation: front support (pushup position) pops, harder variation: handstand pops which fall to her back on a crashmat).
If I see one of my kids is having this problem, I'll also usually make them go back to front walk overs or handstand fall to bridge and standing up with their arms by their ears; I know it's not the same for a front walkover to a front handspring, but it'll get them used to always coming out of the skill with their arms straight by their ears.
The last thing I can recommend is to have her push her hips into an outcurve as she's popping; since she's piking, she might not be able to pop off of her hands because her body's not tight as it goes over. I'd recommend spotting her so she can focus on the hips/arm technique and doesn't feel like she's not going to make it