Perhaps with the smoking some surreptitous use of a camera phone might suffice to convince the landlord to kick him to the kerb.
Or perhaps the time-honoured 'anonymous tipoff' to the local plod that there's some drug (ab)use going down?
The shared house that I lived in a few years ago, had one semi-self contained flat in it, it had it's own kitchenette, but shared the bathroom with the rest of us (who had our own Kitchen).
One of the renters of this "flatette" was a bit of a nuisance, nothing terribly bad, but a variety of tiresome and irritating things, that meant none of us had much time for him, including the landlord.
We found out that he had a predilection for smoking non-traditional substances, and since several of us worked at a large sensitive government establishment, which had made it quite clear that any sniff of substance abuse would result in instant suspension, we weren't too happy about this.
The landlord was informed, who in turn contacted the police and offered them a spare key to nose around the place. They turned up one day, and knocked on his door (the inside door from the hallway into his flat) in case he was in. He was, and answered the door whilst smoking a spliff.