Probably too late, but here goes.

If galvinizing or any coating at all was not in your work agreement, how are you responsible?

Next, electrogalvinize is crap. It should have been sandblasted and hot dip galvinized. Again not your problem, but you can point a finger.

I would work with the customer, tell him that, and then ask for records of the finishing work. Probably the customer is just looking for options. And remind him that next time, you will be using stainless steel.

Did you do the final install? If so you might just be liable.