Thank you for your reply.
I am pretty sure there was no confirmation pop-up, only my character walking on the screen. My hand wasn't even on my mouse, all I was doing is pressing 'S' for my character to move down then suddenly the battle started. I got confused for a while thinking it was a wild pokemon. :D:
I thought friends were trusted and can start pvp one-sided and that the guy added me only to start pvp that way so I unfriended him. :Cry: Pe-something, if you are reading this, I am sorry for thinking badly of you. :D:
I think there need a way to disable pvp. I got pvp invitation from some guy in fancy cloth right after I got my very first pokemon. If this wasn't a bug I am very sure I don't want an accident like this to happen again. Also to avoid potential trolling and people fishing new players.