try this;
take her out back and give her the rod
and reel but do not thread the line.
have her false cast stopping at 10-2:00 continuosly. this will give her a rythm of casting without worrying about the line for now. (10-2 is the most common but not always the best)
after she learns to stop for a second or two then line the rod and add a fly. make sure the fly is bright enough so she can see it.
pull out line enough that YOU can feel the rod load. (a softer rod is easier to feel it load.) and again let her false cast stopping shortly at 10 and 2 without letting any more line out. about three false casts each time.
you should be able to see if she's casting too quickly or too slow or not stopping. get her to cast at the same speed and motion.
when she gets the hang of this now you can let her cast the fly. watch and make sure she still stops briefly at backcast and forward cast. after the loop straightens than she can drop the tip to lay the fly down.
the longer the cast the more line you'll need, this is where you have to stop a little longer on the back cast to let the line straighten out some.
if you're casting heavy flies or streamers you may want to stop at the 1:00 position this way by the time you start the foward cast the heavy fly won't fall too low that it hits you in the head during the forward cast in case you are a little late.