Yeah you have to stop the stepping out it is almost hard to work much else because it is leading to other problems. She is casting out and her front arm is fully extended almost before contact but that is because she is so far away she has to do that to hit the ball. She also has her front knee bent at contact so she does not have a firm front side to hit against but again if her front leg was extended she would be in line with the shortstop and hitting the ball off the end of the bat. You also see that see is pulling her head out staying on a line might help as well.
Hopefully others have good drills I always just put my glove down behind their foot and watch them step on it a few times but that is not very safe drawing the line is good but as you can see without some kind of instant feedback she just keeps doing it even though she is not on the line. Also have her start out a little more natural and relaxed her elbows should rest comfortably down and make an upside down V her back elbow is way too high to start, she is also pre loaded or at least turned alot when starting her stance have her start more parallel to the plate with her shoulders.