Even though some people with diabetes don't fast on Yom Kippur some do. Before Yom Kippur you and your daughter should consult with a competent doctor and with an Orthodox Rabbi.
The Rambam writes that the major purpose of fasting is repentance, returning to a more spiritual life. Not being able to fast, however, does not preclude the possibility of repentance. According to the Rambam, repentance involves three stages: Regret, verbal admission of the sin in prayer, and commitment not to repeat the sin in future.Just as it is a mitzvah for a healthy person to fast, it is a mitzvah for your daughter to preserve her health.
- Mishneh Torah Hilchot Ta'anit 5:1