I should add that if she can't find the flexed elbow action in sling shot, or at any part in her progression you might have to use a 'constraint'. The simplest is a piece of KT tape fully stretched and applied to the inside of her elbow. The 'pulling' action of the tape will serve as a reminder. If you need something more substantial you might have to move to a cubital tunnel brace which keeps the elbow in flexed position.