I just finished an oak floor for a customer. He is screaming mad about an issue with cupping. It is only noticeable (barely) under extreme lighting near a large window. I explained that the industry standard is what shows under normal lighting conditions since very bright lighting (ie: direct sunlight) will show the tiniest imperfection. Any thoughts on this?

It sounds like a minor issue.

Here is a quote from a May 2003 article published by NOFMA to help back up what you are saying. I would point this out to your customer.

"Remember, with any wood
floor, the standard of acceptance
is: “Inspection should be done
from a standing position with normal
lighting. Glare, particularly
from large windows, magnifies any
irregularity in the flooring should
not determine acceptance.”

The full article:
www.nofma.org/Portals/0/...

Good luck!