Suppose that someone has a Student "Titre de Séjour" that is valid for a little more than a year, and this person is in France right now. If this person leaves the European Union for about a month, does the French government, upon the return, have the authority to ban the entry based on COVID-19 grounds? For instance, if this person is coming from a country such as the United States or Brazil, that are more hard hit, can the EU forbid their entrance in the Schengen Area?
I understand that the situation right now might not be this one, I am however supposing a theoretical second-wave or worsening in the given person's home country (USA, Brazil).