Someone, usually of the other sex, that spends a lot of time with one of your friends, hogging them away from you. Friend stealers usually become that person's boyfriend or girlfriend.
Dude 1: "Dude, how come he's not hanging out with us anymore? I've been calling him up all week!"
Dude 2: "He's with that new chick dude, they hang out like every day son!"
Dude 1: "That no good friend stealer!"