Ttrp is an intensely social activity and, like all social activities, involves a constant dynamic of push and pull, always dilating and constricting your range of acceptable inputs and outputs. Exactly how much barging-in-without-scouting are your fellow players going to tolerate? Depends on their natural predilections, the character they’re trying to play, the game they envision in their head, how much they personally like you and the length and quality of your shared history, whether their spouse yelled at them that morning, etc. Social generosity is always being balanced against self-care and -integrity.
Unfortunately the type of people who are most enthusiastic to play ttrpgs tend to be the type of people least able to read a room and manage this dynamic. I think that’s often why they’re attracted to it - the layer of mechanical rules and geek context security blanket gives them a structure they can practice and experiment in, without all the crushing anxiety that might come from a more chaotic and freeform social situation like a party. But that puts one in the position of being these people’s guinea pig. Not fun real quick.
I dunno - at some point you just have to decide your tolerance level for other people’s bullshit, and if the friction grows too hot, have a talk. Unconsented pvp is a big old red line for me, any reasonable person should be able to understand that. If this guy (and it’s gotta be a guy right) won’t get it, you’re within your rights to make it a “me or him” situation.