Two keys here. First, most people would not go as low as she does, yet we've done this for 3+ years and she's never had any patellar tendon issues. That said, she's basically in a full squat position at the bottom of the HT; most athletes can/should be in 3/4 position.

Second thing is to let the ball bounce, then "go down with it" and then throw it has high as you can.

She's throwing a 6 .lb ball, but to be honest, a 4k or 8.8 .lb would be appropriate for her.