Wife is convinced 5yo is “out to get her” how do I dissuade her of this notion?

Title actually pretty much sums it up.For some reason my wife is absolutely convinced our 5yo decides not to finish dinner or fights with his sister for the sole purpose of making his mother’s life harder. As though that is his driving motivation and not that he doesn’t like carrots or was irritated by his sister. She is convinced he gets satisfaction out of seeing her upset.She’s also become convinced he’s trying to manipulate her and “gaslight her.” I told he’s incapable of that level of social understanding and just lies because he’s going through a phase (e.g., she’ll ask if he brushed his teeth. He’ll say yes even when he clearly hasn’t moved from the same room for hours.)Obviously behaviors we work on, but not anything out of the ordinary for a 5 yo and definitely not derived from the desire to convince his mom she’s going crazy.Our 8yo did all these same things at that age and I she didn’t have this reaction then. She doesn’t have PPD and has never struggled with any sort of mental illness.Not sure where to go from here but getting concerned. via /r/Parenting http://bit.ly/2EzUBL7