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I briefly researched the issue. I found no cases where a Mexican marriage certificate was used as a de facto pre-nuptial agreement. But as I was reading your question, before I got to your mention of the possibility of using it as a pre-nup, I thought, "I bet they could argue that the marriage election is a de-facto pre-nuptial agreement."
The marriage is valid in California, but the Mexican laws would not bind the divorce court... so the default position would be that the property is marital property. But she could overcome that presumption by either showing that the property was "separate property" under California's definition (brought into the marriage, inherited, etc.) or that the election on the marriage certificate should act as a pre-nuptial contract.
I think it would be a case of first impression (though I cannot promise I did not miss a case)... so she could argue anything she wants. It sounds like a solid argument to me.
She could also try to get him to sign a post-nuptial agreement now saying that he has no interest in the property. But if they are on bad terms, that could backfire.