Just jack up the rent on each unit to cover the cost of their usage.
What else would you recommend?
It's tricky, he raises the rent, then his numbers look higher than others in the listings. And a big 'free laundry' add will invite more abuse. Building security should be reviewed, including gate options, but he needs a way to collect extra value without putting that value in the room.
Perhaps a machine with some kind of electronic value or a coded door with subscriptions for the door. But it looks to be a tiny room, so probably a tiny complex. Not much budget then, for getting fancy. Have to find a better lock box or mount it inside a cage.