Sir this isn't reddit.

It’s not about religion. That’s just yet example they chose. My opinion on the matter is unchanged.

You're not wrong. She thought he's a proper orthodox Jew, he probably wasn't for a long time if ever, that definitely qualifies as an illusion. If hypothetical she were to leave, perhaps his illusion might have been that she'd not leave, but accept his decision and find a way to make it work regardless. Is she leaving because he kept his doubts a secret? I'd get that, can't have a working relationship without honesty. But if not: It's not like that sort of thing working out is completely unheard of, and while the HN audience tends to be dismissive of anything that doesn't feel simplistically rational, love is a pretty powerful thing that has overcome worse obstacles time and again.