[James]

Most likely, there is something wrong with the car antenna connections,
either at the radio itself, or at the chassis point where it is mounted.
It sounds like the antenna isn't property grounded.

I would first have him check and double check the plug point where the
antenna plugs into the radio. Make sure the plug is pushed all the way in.
Also, be aware that on many car radio setups, there is a 2-3 foot antenna
jumper wire which terminates in a female plug, and then then car antenna
male plug fits into that female plug. That joinder should also be checked
for tightness.

Lastly, the antenna mount should be checked , to make 100% sure that it is
grounded to the car chassis. If possible, remove the mount, scrap the paint
or crud from the car body, and remount the antenna.

Hope this helps. Please report back.

James