Don't be too quick to drop acustomer particularly not one in a nice little group.
Although we all trust WFP and with good reason, it's not impossible for us to miss a bit or not rinse properly.
Call his bluff ask to see the ones that are wrong.
I always do that with complaints, I do it in a nice way, something like 'well that doesn't sound right can I see?'
That also puts paid to those dodgy types who use complaining as a negotiating tool.