No, only individuals of legal age are permitted to enter a civil union (Art. 515-1 C. civ.). Additionally, as it involves a contract, one must also possess the capacity to contract. For persons of foreign nationality, their legal capacity is determined by the laws of their respective countries (Art. 3, para. 3 C. civ. and Cour de Cassation, 1st Civ., June 6, 1990).
Therefore, if your friend does not meet the age requirement for a civil union in his or her own country, he or she will not be able to do so in France either.