Anyone who ditches or lets their friend down, usually to do some generally undesirable task, especially if it is taking blame, exhausting work, getting in a fight. A buddy fucker usually takes a lot of blame from his friends, and depending on how bad the ditching was, may not be friends with the fucked buddy much longer.
Joe: You wanna fight?!
Bob: C'mon Jim, back me up here. *Notices Jim is gone and gets beaten up*
The next day...
Bob: Jim, you are such a buddy fucker