What if they sugar coated it a bit? Say they waited for you to have said wizard patron do something suspicious, perhaps even very subtle, but since they are looking for it, they seize on it as a reason to be paranoid.
Originally I wrote a reply to this but apparently it got eaten at some point and I never sent it.
Taking this seriously and trying to give full doubt benefits: it might work, but I'm still of a skeptical mind about this. That is, if you're on such a hair trigger that something "very subtle" makes the character prepare for full-on betrayal, that still sounds like pretty extreme paranoia. Sure, it's paranoia that waits for some kind of justification, but it still seems like an extreme response to small stimuli.
Now, trying to give benefit of the doubt, perhaps this is more mild than it seems. E.g., let's say the character is someone who doesn't trust easily to begin with, and takes that "very subtle" suspicious action as a sign to dig deeper. They go looking, and do minor preparations (e.g. counting coins to make sure everything is paid up correctly, keeping careful records of materials delivered and performing meticulous examinations of goods received.) They don't immediately leap to cutting the person out of their life as much as possible and making ready to be betrayed, but rather
do work to determine
whether there's grounds for their "very subtle" suspicions to be borne out. If that's the case....alright, I can probably deal with that, but the player and I may have a conversation later on about whether they are intentionally playing certain types of characters because they're being "DM genre savvy" as it were.
More importantly, if it really is that big a deal, I can always tweak things. Perhaps the Wizard
is intending to betray the party, but their earnest good-hearted deeds endear them to her. Her suspicious activities thus become more her trying to pull out of her efforts to betray them. Then, the player playing the hyper-suspicious character can still gather all the clues, assemble a case, and blame the Wizard, and the party can explore a different story, hopefully one that will be more fresh and avoid such blatant predictability that the players could see it literally the instant they meet a new character.
However....that's gotten pretty far afield from the original example, at least as far as I see it. The original phrasing was, "[The DM] loves surprise face heal turns... I bet this wizard that is hiring us will turn on us better prep for that." I don't really buy that a player will be able to, as you say, "sugar coat" it with a veneer of roleplay without pulling it far enough away from the original phrasing that it's fundamentally a different beast. E.g., if the player simply
thinks that to herself, and then has her character operate as a reasonable, sensible person within the confines of the story, it just doesn't sound like she
could do the thing described, preparing for a "curse your sudden but inevitable betrayal" moment the instant after meeting their new employer.