The standard working week is 35 hours and any hour worked over and above 35 hours would normally constitute overtime and paid at an increased rate. Alternatively, rest days (RTT) should be given in lieu in accordance with the provisions set out by the applicable collective bargaining agreement.
That said, the recent changes in Employment Law (which tend to extend the scope of collective negotiations) allow the employer to provide for more flexible provisions such as annualised working time in terms of days (in French “forfait jours”).
Thus, it would be important to verify the provisions of the applicable collective bargaining agreement or in the event that there are no provisions in the CBA in this respect, the employer could agree to a company-wide agreement which can now be implemented even with only one employee in the company.