"A spoonful of sugar ..."
This is really only a Contract Review situation. There are a number of non-confrontational methods to accomplish what the supplier really needs:
"written documentation of the changes approved by someone in authority at the customer."
The sorry fact of life is that many customers are woefully delinquent in maintaining up-to-date documentation. It is certainly within the purview of a good supplier to mark up the verbal changes (red line) on a customer's engineering drawing and send them along to the appropriate person (supplier should know, but if not, just phone and ask) for a sign off.
Smart suppliers send a copy of the signed off drawing along with the shipment to assure the customer's incoming inspection is working with updated drawings.
If the sign offs don't return in a timely fashion, then, and only then, should supplier phone and say, "Hey, we need the signed-off copies or our bean counters won't let us risk raw material and time on an unauthorized version. Can you fax over or email a copy right away?"
"Customer centric" means making life as easy as possible for the customer, NOT playing "GOTCHA." suppliers want customers to treat them like partners, not adversaries and vice versa.