what happens when a tennant switches it on? will that be there fault too?
sounds to me like he's trying to pull a fast one, i'd offer him compensation for the electric you used and if he don't agree to that then, see ya in court landlord.
its his responsibilty to make sure all electrical appliances are safe, not just for his tennants but for people he employs to work there too, you've done him a favour by bringing this fault to his attention.
derek