Florian Meier has a great video [which I can't find since I don't know what keywords to use] where he put a bunch of balls in a plastic bag, tied up the bag, and then put it in a bigger plastic bag with long handles. He then had the student grab the handles and swing in a service like motion. In order to do it smoothly without hitting yourself in the head due to jerky motion, you have to swing it with a reasonable approximation to a serve.
Here's one example, although it's not the one I was hoping to find [the guy's motion isn't as full circular as I would like but it's the right idea]:
Another idea: video yourself throwing and compare with a video of her throwing so she at least understands what the differences are even if she can't fix them yet.
Then, guide her arm through the correct throwing path with proper elbow separation. Do that enough times and she will have the muscle feel for how to do it. Then let go and let her body take over. Sort like throwing with training wheels. The problem with just telling her to throw is that she doesn't know what the proper motion is supposed to feel like so her trying to do it full speed with no guidance is difficult. This way, you're making her follow a proscribed path, even if slowly at first.
Also, are you sure that her shoulder is bio-mechanically sound? Maybe she has some shoulder issue that limits her range of motion.