I learned to always shut off the water and open a faucet to drain because if the by pass valve that hasn't been used in years doesn't by pass you are trying to take the by pass off a pressurized softener and can break the clips and have water going everywhere until you go shut off the water. It's like wearing a seat belt although you are not planning on having an accident.

Once the control is off the by pass he can turn on the water and see if the by pass leaks and if not leave the water on.