I second moving back to goblets but suggest doing them a bit different.
With the goblet squats, it's very easy to sit back and be vertical when you hold the weight out in front of you. It's a counter-balance.
Have her hold a dumbbell with the top end sitting in the heels of her hands. This is more comfortable and stable than wrapping your hands around the handle and "iso-curling", which allows for more weight/reps.
Doing them this way puts the weight closer to the heels and forces the body to maintain it's upright position more actively. The heavier you get, the more it feels like the DB is pulling you forward.
I would also suggest putting a mini-band around her knees during these to force the glutes to fire and strengthen as well as facilitate learning "knees out".
Edit: After watching the video again, her ankle mobility needs work. I can see her heels coming up midway through. Also, work on sitting down between the knees, not behind. Full range with the goblets should help this, but you have to have her go ALL the way down, as if she was just gong to sit there for a while.