Jews not according to Halacha

Hello Rabbi. Yesterday, a good friend called me who is very right-wing but distinctly secular. He told me that he had been dating someone for a few months who was eligible for the Law of Return but was not Jewish according to Jewish law. He insisted that it was an amazing relationship and that they broke up because of the halakhic part. She had taken a course in the army but at the last minute decided not to immerse herself in the mikveh, claiming that "God asked her not to." He asked me, as the religious friend, to convince him why he shouldn't go back to being in a relationship with her and why it was terrible that she was a gentile, etc. The girl said that their worldview was quite similar. And given that he was a Jewish nationalist, that meant that she too saw herself as Jewish in a national sense. I told him that if Jewish law didn't interest him anyway, I wondered why it was such a deal breaker for him, and that if Merav Michaeli had been flexible and had a child through surrogacy, I found it strange that he wasn't willing to be flexible for the sake of a relationship. I explained to him at the beginning that this was a sweeping halakhic prohibition and that there was no way to allow it, even if his children converted in the future. I was really horrified by the answer I gave him because in a way I had convinced him Him to assimilate (he also kind of said that). But what could I say back to him?
Anyway, after the conversation, I sent for a rabbi who I really respect, but I still feel terribly, terribly confused. What would you answer to a question like that? And what should I do to make amends?
Thank you, Noam.