First, a tankless sized to provide hot water to 2 units will probably require a 240 volt, 120 amp circuit. Your 65 gallon unit probably has a 30 amp circuit, and might at most have 40 amps, so if you even have a large enough service to support that, there will be a pretty large bill to install the circuit. Your best solution is to get rid of the tenant who for some reason feels it is ok to turn off that breaker!
WHo pays the electric? If #2 pays the electric, then of course in his place, I would also flip off the breaker!