What must a sister do who broke her fast in Ramadan half an hour before Maghrib because she did not hear the Adhan in France?

It is permissible to break the fast based on the strong presumption that the sun has set, and certainty is not a prerequisite. If a Muslim breaks the fast based on this presumption and then it becomes clear that there was an error, the majority of scholars hold that a make-up fast is obligatory, while others believe the fast is valid and no make-up is required.

As for breaking the fast half an hour before its time, this is a clear act of negligence and necessitates repentance and making up the fast. One should be diligent in ascertaining the correct timings for prayer and fasting.