Seek a COMPETENT repair shop. The shop you described cant be competent.
There is usually only a simple 3-screw clamp at the front just under the filer/name ring that connects the focus ring to the driver helicals. Somehow the screws lost their grip on the clamp. How the screws loosened is puzzling. Nikon applies a dab of lacquer to the side of these screw heads where they meet the clamp. When it hardens, it immobilizes the screws so that they cannot loosen unless forcibly turned with a screwdriver.
If the lens is used, it might have been disassembled by someone who failed to reapply the securing lacquer allowing the screws to eventually loosen. This is really a simple job. Since the clamp has loosened, the infinity focus will be lost. That has to be re-indexed just prior to tightening the clamping screws and securing the screws with the lacquer.
On a few lenses the clamping screws are under a rubber gripping ring on the barrel. Ive had no reason to disassemble my 105/1.8 so Im unsure of the location of the screws for this particular lens.
This should be utterly simple for a camera technician worthy of the title. Look elsewhere for competent service. If the focus is stiff, this might be the time to have the helicals cleaned of old, dry lube and fresh helicoid lubricant installed.