Well a muslim man is permitted to marry a Christian/Jewish woman, it is permitted by Quran/Islam. So a blanket statement that he cannot marry a Christian/Jewish woman cannot be made.
Now what kind of scripture are they following and how truly Christian or Jewish are they is a entirely seperate issue and perhaps upto the individual Christian/Jew, we cannot make a judgement about all of them in one big sweep. There are sects of Christianity who believe in the oneness of God and consider Jesus just a messenger/prophet.
But to make a statement that a muslim man cannot marry a Christian/Jewish woman would be contradicting the Quran.
God knows best.