What kind of lean are you trying to fix? is it a lean towards 3rd as a RHP? Or a lean towards the catcher? I am of the opinion that the 3rd base lean is a direct cause of kids feeling the need to create space between their hand and their body as they release. Show me a leaner and I will show you a non-brusher. Some say that if you fix posture then brush happens automatically, I have found that if a kid has it in their head that they need to avoid all contact with their body then it creates a lean.
To fix it just have her do K type drills with perfect posture as the only goal and letting her arm get as close to the body as possible. Once she realizes that she can still throw with good posture and a little contact then she is on the way. The other benefit her is that from this position she has to lead with her pinky or she will whack her thigh with the ball so hard it will hurt. After a few whacks she should get the feel.