It is important to stress proper throwing mechanics every practice. It is the first thing we do after our dynamic warmup. We progress from wrist snaps, to one knee, to check throws, rock 'n fires, full motion, and front step. Sometimes we throw some extra drills in there to mix it up, but these are the basics we do every time. BTW, even the Olympians do the basic, fundamental throwing drills every day.
Here is a link to the ThrowMax website. It is something I have some girls use when they just refuse to get it. The links on the left are informative and include some drills you can use.
I used the throwmax on my DD and it worked. They do not like it because when they throw wrong, it hurts.
I was told that when girls are young their hands sometimes are not big or strong enough to grip ball properly and they shotput and then just get in the habit of this. A smaller ball and teaching mechanics right can help alot. I know this is not your case but just some info to pass along.
I haven't tried the throwmax but it looks like it would do the trick.
I also work with the girls with fundamentals and I find that throwing from one knee and throwing from the crane position are excellent for this. If you don't have a throwmax you can start with these right away.
The Crane is where they start standing sideways to the target pointing either glove or glove elbow at the target and holding the ball with upper arm parallel to ground, elbow bent 90 and palm facing out. you have them lift the front leg and hold it for a 2 count and then step forward with it while turning at the waist and throwing.
Girls natural tendency is to face the target rather than turning sideways to the target, so you want to reinforce this sideways orientation. Throwing small or Jr sized football is good for this too because they really need to emphasize the sideways starting position and the turn and follow-thru in order to make a decent throw.
The good news is that she hasn't been playing long so the bad habit isn't cast in stone yet. When they're 12, been playing since they were 5 or 6 and still do the shot put (especially if they've gotten better at it) that's when it's really hard to break them of the habit.