Steal it back.
Actually, I hope he got some peripherals and stuff with it, because you can buy a new one for 150 now, and a refurbished one for around 100. You may want to lower what he owes you if all he got was the system, it would be more fair overall and would probably make it more likely that he'd pay you back.
And what kind of a friend doesn't pay what he owes, anyways?
