A guy I know had his bmx stolen. A few of us left are bikes outside a kwik-save with out putting any locks on them (i know its really stupid). When we got back only his bike had been stolen, which is strange considering his looked knackered and rusty compared to the rest. But I had no sympathy for him because he was a proper klepto anyway. A fine example of Karma. However in you're case should karma fail to show up, I agree with most of the others. Threaten him and if that fails Hit, Kick or otherwise maim the little tosser untill he coughs up a bike. Hope your friend gets his bike back, Good Luck.