Walking out on three month notice = bad reference, no holiday benefits etc, getting a reputation as unreliable.
Getting fired = bad reference, no holiday benefits etc, getting a reputation as unreliable.
The best option would therefore be to not turn up to work, as this will achieve both of the above, simultaneously, with zero effort. Any other efforts to get fired would be pointless. Unless, of course, he really wanted to get a reputation as unreliable and a jerk. But I can’t imagine why anyone would set out on purpose to do this.
You might want to consider how your boyfriend’s new employer might look upon it. Will they be happy knowing that they’re the next one’s to get imaginatively shafted should the circumstances arise again? And is your boyfriend going to remove his current job in his CV forever, just in case a future employer phones them up for a reference? Is his friend going to be always there to cover for him? Is his friend sure that his job is safe if their employer ever finds out about their scheme? Do you think their employer is stupid and won’t work out that he’s done this deliberately? What’s more likely to get him taken to court; walking out early or intentionaly screwing things up?
Sorry to be the killjoy, but I don’t see how messing up on his job to get fired, and probably harming his present employer in the process, is in anyone’s interests.