He knew this because their (Jewish) gf’s friends and parents disapproved of him. We explained why these people would not disapprove of him because he had been a Christian dating a Jew, which is another issue altogether because he was Christian; they disapproved of him.
Conventional Judaism doesn’t allow marriages that are interfaith. The Torah states that the young young ones of these marriages is lost to Judaism (Deut. 7:3-4), and experience shows the reality with this passage all too well.