Torque convertor balance questions

If it was a weighted imbalance it would vibrate all the time! At idle does it vibrate enough to feel the vibrations by simply placing your hand on the fender! It would be a harmonic vibration, and would be happening all the time!

If it is happening only between 40-50 mph, it is in the driveline! He needs to start by eliminating all of the driveline components one by one! Don't discount the rims and tires just because they are new, check everything! If it's possible, have the wheels and tires checked on the car, it could be a bearing in a wheel hub!