What is the ruling on a person breaking their fast on the thirtieth day of Ramadan, claiming to have seen the crescent moon in another country?

If a Muslim is in a country that relies on the Shariah-compliant sighting (of the moon) to determine the beginning and end of the month, then he must conform to them in fasting and breaking fast. However, if he is in a disbelieving country, or a country where they manipulate the determination of the month without observing the Shariah-compliant sighting, then it is permissible for him to follow those whose sighting and adherence to Shariah rulings he trusts.