For a resistive load, the power will increase with an increase in voltage (P=V^2/R). The increase in power from 122V to 133V is about 19% in this case. Reducing his bill by 19% still gives close to $380/month, though. It sounds like something else is wrong, or his power usage is much higher than he claims.
Some loads will maintain nearly constant power regardless of voltage, but I don't know of any that would draw less power at higher voltages. Perhaps you are thinking of power line loss, which is lower at higher voltages because the current is lower for the same power.
