I occasionally buy gold. Over the years, I have come across a lot of plated gold, but rarely is a plated piece karat stamped.
Based on the data you have described, it sounds like the rings are made of 9kt and plated with 14kt.
You could do the XRF as Ken recommended, but it is possible that no amount of independent testing will keep this customer happy given that he already has an independent opinion from his expert that thiese rings are 9kt.
I would just remake the rings out of 14kt to keep the customer happy and save the company’s reputation or at least minimize the damage to the company’s reputation.