Not sure what she means by "hold back on the salt for now" in directions. If she means a little salt now, and some later, then it may (according to some poskim) allow her to leave her prepped chopped onion overnight in the fridge. But if she means to add no salt, then she should not be leaving chopped onions overnight, thereby proving that her geirus was a sham.
You're not serious about calling her geirus a sham if she leaves onions overnight, are you?
The problematic parts of the recipe is not putting it up until the night, and adding salt to the pot in the morning, but the recipe is not geared only for Shabbos, or for Jews, for that matter, so although some halachic disclaimers would have been nice, I don't think this is a big deal.