She will probably keep changing her mind for the next few years. If she really is 15 I'm sure she will eventually leave Islam. She's already come farther than most non-Muslims will in understanding what Islam really is.
Remember this, too:
If one of us officially announces our intention to leave our religion, the worst we'll get is a bit of grumbling from a few relatives.
For a girl raised in a muslim home in a muslim community (like Sarah), she faces screaming fits, threats, beating and other things from her own family. From the broarder muslim community, she faces stalking, assault, rape and murder!
Making an official disconnection also entails rejecting EVERYTHING in her life and upbringing. Religion, culture, family, education, school friends etc. Any surprise she gets conflicted from time to time?