It is very sick but funny. The kind of joke I'd love to play but wouldn't really.

I would like the prankster to get paid back though.

Best possible outcome would be if the flat mate saw the dead body then immediately phoned the deceased's girlfriend and say's 'Babe this is great, the cunt has hung himself we can finally be together properly now! I won't have to kill him after all!, well anyway I'm just going to leave him hanging here for tonight and I'll come round and fuck you now. I'll get some champagne on the way so we can celebrate! Love you, see you soon xx'

That's how I'd ideally like this prank to end.