1 cup in 90gal = .07% (7 one hundredths of one percent)
I would be shocked if this would cause big trouble in an engine that sucks a full gallon of fuel through in 6 - 8 minutes, going down the highway. If my math is right, 5oz of fuel goes through the engine in about 16 seconds at highway speed..... Everything also goes through both water separators and fuel filters on the way.
while it certainly makes sense to err on the side of caution, I'm thinking this was a bit of overkill. JMHO.........
HOWEVER.... I'm still trying to understand how it's even remotely possible to confuse the diesel and water tank fillers........ That one has me scratching my head. ;-)