Well it depends on where you feel she is not improving in technique. I think the root of this students problem is she approaches music by sight reading through the entire piece without understanding the essence or technical challenges of the piece before hand. This is a time for a sight reading and thiere is a time in practice for learning a piece and this student needs to understand that sight-reading is not learning a piece!
Sight-reading at the end of the day is skill that simply makes music reading easier and quicker but what it is not is not a through clear and understanding of the piece .
She needs to completely change the way she practices.
First when she sees a piece particually difficult pieces she needs to from day one start working on the most difficult areas of the piece or places that require a specific techniques such as wrist rotations, wrist staccatos, fast passages that requiqre strong finger tip or raising the wrist to change the color of the notes. Use techniques procedures to accomplish music goals and you as the teacher have to lead her to pieces that she cannot sight- read and show her ahead of time how to do this. The solution cannot not be to take steps backwards and give her pieces that are not challenging for her read but she is forgeting to apply the technique. You have to infuse the technique from the very begining so it can become ingrained her playing of the piece. She will auto matically always perfrom the technique because that is what she has done form day one.
She should start thinking of the mood of the piece, tempo, character, intention, melodic and rhythmic direction.
She should recogniqe tonal and harmonic relationships, key changes, important chords
She needs to understand form, differnces in structure, dynamic differences, diffent textures, voices and nuance. There is no way you can play all of that just by playing from beggining to end. For example if you give her a concerto or Chopin etude, unless you are Franz Liszt or have an incredible amount of experience you are not going to be able to simply sight-read it all the way through.
As far as her not being bored by being able to sight -read through the material.. change the material! You can find pieces that it would be impossible to read through with out overcoming difficialties first!
This is coming from someone who was is a confessed sight-reader so I have seen the usefulness of sight-reading and well as its short coming and it is far more difficult to ingrain a piece where you used the wrong technique for so long and then put in the right technique after playing it so many times the wrong way. I suggest you give her Bercuse by Chopin. It is a beautiful piece that is decepitvly tough and diffcult to sight read but incorporates many pianisit techniques ..Good luck!