vibrating steering wheel

First check the wheel lug nuts are tight. Then look for signs of a wheel weight to have come off since the wheels were balanced. What does your son mean by "difficult to drive around town"? I would suggest going back to the people that did the work if possible and ask them to drive the car and see what their impression is of what is wrong from just a test drive.

When do you feel the vibration? With the brakes applied? What speeds?

I agree with Patrick. Check the lugnuts first to see that all of them are tight. Then check to see if there is an outline of any missing wheel weights. Unbalanced tires could account for a vibrating steering wheel. My best guess is that the front end was not realigned after the balljoints were installed. That would account for the difficulty in driving around town (incorrect tire angle causing tires to dig-in to the pavement.) One question to ask your son: is the steering wheel moving left to right or up and down. The first is tire balance. The second is front end alignment. Also check to see that the front brake pads are not siezed against the calipers. Dennis