Do i need a backflow preventer for drip irrigation system
I think he is mistaken.  A pressure reducing valve is not listed in the code among the approved backflow prevention devices:


  2009 Uniform Plumbing Code 603.4.6 Protection From Lawn Sprinklers and Irrigation Systems
  
  603.4.6.1  Potable water supplies to systems having no pumps or connections for pumping equipment, and no chemical injection or
  provisions for chemical injection, shall be protected from backflow by
  one of the following devices:
  
  (1) Atmospheric vacuum breaker (2) Pressure vacuum breaker (3)
  Spill-resistant pressure vacuum breaker (4) Reduced pressure vacuum
  breaker