I've had something similar happen. I was waiting for one of those world boss spawns for the last part of a bonus quest line. As I stood there waiting, some guy came up clearly needing the kill aswell, so I invited him to a group. He kept declining the invites for no reason, as he just stood there waiting near the spawn area, and eventually stole the kill right under my nose... Talk about being a douche.

Oh god.. been there done that. I had a "friend" just like yours in another mmo who just wouldn't leave me alone either, he was online all the damn time, and instantly pm'ed whenever I came online on any of my characters he knew about. I ended up getting bored of the game and took a 6 month break, returned, guess who messaged me the second I came online? That's right.. an alt of his I had never heard of before.. he had apparently been religiously adding every one of my characters to his friends list on every alt, for half a year!