My armchair speculation, based on interactions with some genuine freaks, is that she secretly is shit scared of making a fool of herself. In front of you, in front of others, in front of herself. And I really mean scared - she doesn't talk about it to anyone, doesn't even think about it, just avoids this issue altogether. Every possible action, every idea which leads to thoughts of failure is rejected.

Remember it's not only quarter million bucks, but also her romantic feelings and her faith in this scammer and people in general. And her judgment and common sense.

So by some stupid fluke she took this risk and now she will do everything to make it work and prove she's right. The scammer offers her a path forward by promising happiness ever after, you guys don't because you keep telling her she will fail. This is roughly what I meant by "appearance of being treated seriously".

Probably she had never been that much of a fool, maybe she heard people laughing at others who had, it's entirely possible that it just doesn't occur to her that anyone could be understood and respected after such a spectacular failure. Especially if all the way along everybody was telling her she's wrong. Now if she fails she expects to be alone with it and have no one to understand her. So she can't afford her marriage plan to fail and will systematically disregard all of your advice which would kill this plan.

So what can you do? Don't listen to me, I may be totally off the mark. Don't care too much about her words, don't argue, instead pay close attention to her emotions. BTW, don't expect her to acknowledge them verbally if you catch her hiding them, it's going nowhere. Figure out what she wants, what gives her hope, what makes her anxious (probably most of the things you've been doing so far). Let her talk to heart's content about her plans regarding this scumbag and don't distance yourself from them (they are bogus anyway), let her figure it out on her own. She already has all the data to decide, but refuses to think about it. Don't treat her like a child by repeating arguments she could already recite from memory or expecting her to say one thing or another. It's her internal thoughts that really matter and you know none of it.

Think. Get help from a pro psychologist if you don't trust Internet smart-asses. Disregard anything that doesn't apply, it's all based on my observations.