Perhaps this area is considered exposed to SEVERE physical damage. That usually ups the restrictions. Will this conduit be buried, laying on the ground, or like a "guard rail"? Are the conductors service conductors (art 230) or other conductors?
But in searching the 2005 code for "severe physical", I only get a handful of hits. The closest thing that may be confusing him is 388.12 for surface non-metallic raceways. But I don't think sch 80 PVC is considered a surface non-metallic raceway.
He could also be reading 352.10(F) or 352.12(C) which prohibit RNC from use where exposed to physical damage. However, there are many places in the code that specifically permit the use of sch 80 when exposed to physical damage. Iwire provided one source if these are Service conductors.