What the slob did has nothing to do with whether it was her assigned seat. If she was infringing on OP's space or possessions, it either stops with a polite ask or one invovles a FA or one moves on.
The seat poaching issue is entirely different and is fundamentally none of other passenegrs' business. Yes, in a broad-based way it cuts into revenue and thus causes others to pay more for the same PRASM, but that is a bit strained. Unless it affects safety or security and I am getting the service I paid for, I don't play hall monitor.