On the other hand, that line you showed there is not a thought bubble, they're words MC said directly to Maya. She neither follows up or appears to heed the warning (yet).He can straight up tell her that Quinn offers sex for money. The MC definitely knows that much and it's far more specific than just saying she's 'evil.'
If he turns down Quinn's offer anything more is speculation, but not all speculation is created equal. The way Quinn describes her restaurant does imply the menu consists of more than just her. If we assume there are other girls, the HOTs are the obvious suspects. I mean who else is there? They'd have to be girls Quinn knows, and teachers or professional hookers wouldn't make any sense. Even if the menu included non-HOT students, you'd expect the HOTs to be disproportionately represented simply because of their reputation and Quinn's social circle.
In fact, if we look at what a pure CHICK MC says when he tries to warn Maya, it seems he has indeed made that connection:
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So no, I don't think we can excuse the MC's lousy warning on lack of character knowledge. Yes, MCs who used Quinn's menu might want to omit that fact (though they could still lie and say they turned down the offer), but by the same token MCs who didn't should be that much more motivated to warn Maya specifically.
With that said, though, I think that's fundamentally realistic: as much as there's a lot going on between MC and Maya, they haven't known each other that long. I don't think it's unrealistic for Maya to more or less take the warning under advisement, but for it to be important to her deciding that yes, something IS wrong with this situation and this is why he warned me, once she ends up in the vicinity of such a situation. Maya doesn't exactly ask MC to elaborate in this scene, so she either doesn't get what he's implying or she doesn't take it seriously yet, and, as you mentioned, he's pretty clear about what he believes.