C reminds me of a toxic former coworker who once let slip she struggles with a personality disorder. Researching the disorder later, the phrase, "persistent lack of self love" stuck out. I figured the only way to counter that would be persistent compassion. As much as she damaged my career, I can find compassion for someone who deals with that kind of pain.
From then on, I started every interaction with a soothing sentiment: "Wow, you did such a great job there," or, "That's awful they didn't recognize how hard you worked!" It was like a switch flipped. She still found ways to cause chaos, but rarely with me, whereas before I'd been a main target. Somehow she never figured out what I was doing. The only side effect was throwing up in my mouth while complimenting her screw ups. Dishonesty isn't my cup of tea.
If you have a goal to reach with C before you part ways, I highly recommend this approach.
Examples would be, "Wow, your bird netting is...going to make birds think twice." And, "Gosh, it must be difficult dealing with all the ways people are accusing you of stealing." <--Kidding. You have to really make her feel seen. She knows she's screwing up but can't stop until someone sees her. I worry for Henry.
