Because Nick in fact did not post the Cathy Cluck
Magnar is using the Brawler app as a back door into cell phones and computers. He waited until someone at the party signed on, which just happened to be Nick. Detecting that Magnar remotely logged in, made the post, while Nick is sitting there with the laptop. Nick see's the post but knows he didn't make it, but he's the one with the laptop, alone, with no witnesses to what actually happened. He panics, tries to hide the laptop and gets semi-caught by the MC who doesn't realize until later Nick was hiding the laptop so people wouldn't know he was using it at the time.
But Nick didn't make the post. He was just the unlucky sap who happened to use the laptop at the time Magnar remotely made the post, leaving all the blame pointing at Nick, who is actually innocent.
Doesn't cause me to have much sympathy for Nick because of how he acted afterwards. He hid the laptop instead of going to his brothers and telling them what happened (which would have made clear someone else was remotely using their account and likely revealed Magnar's tampering much sooner and collapsed his plot much sooner). When he was caught he threw everyone else under the bus, revealing things told to him in confidence. He may be innocent of most of the crimes he's accused of, but that doesn't make him any less the rat.
I'm not sure which BaD character she maps to the most but that would probably be my pick
