Never mind the hair, if you have metal drainage the acid can interact with the pipe and cause corrosion and resulting constriction as well as rough spots for further clogs.
Bottom line..NEVER listen to the "guy" at the hardware store, home depot, or any retail DIY supplier unless you know them personally and know they have experience.
You now have two options...
1. Take the drain cleaners word for it and pay.
2. Try another drain cleaning outfit and hope they have better news.
The next drain cleaner would likely charge close to what you were already quoted for a price to replace the drainage anyway.
Chaulk this up to an expensive lesson, also, whatever you do..make absolutely certain the next guy to work on it knows there was acid used, it could cause severe injury if he isn't aware.
FYI, the price given to replace the lav arm is pretty cheap.