Who is correct regarding the time to abstain from food, and is the action of the young man who drank water after the first Fajr adhan and before the later adhan considered acceptable as an attempt to apply the Sunnah, especially since it nearly caused discord in the mosque?

The criterion for abstaining from food and drink is the rise of the true dawn (Fajr Sadiq). So, whoever drinks after it has erred and must make up that day. As for whoever drinks before it, there is no blame upon him. There is no harm in him drinking in front of people if it is permissible for him, and he should not be prevented from permissible things due to people's ignorance.