Is it permissible to marry a Christian woman who does not believe in the One God, knowing that she is not convinced by her religion and desires to embrace Islam for the purpose of marriage?

It is not permissible to declare marriage to a Christian woman as forbidden, because Allah has made it permissible under certain conditions. It is permissible for a Muslim man to marry a woman from the People of the Book (Jewish or Christian) if she is chaste (muhsana). The decision to marry this particular girl rests with your relative, and if he delays the marriage until she embraces Islam, that would be good. He should not rush into this matter and should seek counsel and perform Istikhara (prayer for guidance).