All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
According to the consensus of scholars, a non-Muslim does not inherit from a Muslim. One of the texts supportive of this ruling is the hadeeth reported on the authority of Usaamah ibn Zayd in which the Prophet, sallallaahu ‘alayhi wa sallam, said, "A Muslim does not inherit from a non-Muslim, and a non-Muslim does not inherit from a Muslim." [Al-Bukhaari, Muslim, and others]
An-Nawawi commented on the hadeeth, saying, "They (Muslim scholars) unanimously agreed that a non-Muslim does not inherit from a Muslim." [Sharh Saheeh Muslim]
Based on the above, if this Christian woman's son is non-Muslim, then he is not entitled to inherit from his Muslim father.
However, this matter should be investigated further and the case should be referred to a Shariah court or an Islamic center to distribute the deceased's estate properly and identify the eligible heirs whom are entitled to inherit.
Allah knows best.