Just take out the screw in the tub's drain theory (before leaving). Even if you screw it again after so he doesn't notice. That simple gesture cause water leaks that'll do a lot of damage and will be extremely complicated to fix. 
It's because that way, the pipe that connects the tub to the bathroom p trap will become loose and the water will accumulate in the floor under the tub instead.
It's because that way, the pipe that connects the tub to the bathroom p trap will become loose and the water will accumulate in the floor under the tub instead.
