Years back poaching with in same class was more common and seemingly more acceptable. I even knew of at least one person who made an art out of poaching even F seats when assigned coach years back, how he was able to get away with it I really don't know. Now with pricing differential through out even the class cabin it has become very taboo. I have of course slid over to and empty aisle seat when assigned a middle seat and the aisle/window remained open post boarding. Is this poaching? I guess so in the strictest meaning of the word. In the OP case it was more than just poaching, the pax was rude and disrespectful. To me this is far worse.