Ok. Ok. The answer was so simple, I was embarrassed that I didn't see it right away. The B key was not closing the C key* because the cork between the top of the B foot and the back bar of the C had moved out of place. This caused every note to play perfectly except B. [because the A key closes the C for everything else]. However, since it was still hanging by a thread of contact cement on the side of the cork, it moved back into place atop the B foot when the customer put it in the case to bring to me.
What the customer said to me was that it was like the B wasn't closing the small key above it. I then put my "pig sticker" into the cork and it came right off. Spike's comment caused me to think of this:
"2B or not 2B---that is the question" which summed it up quite nicely.
* For those not familiar with the terminology, the C key (keycup) is the small one located directly above the B.