I am trying to find some standards/guidelines/etc. for proper piping materials in a clean room environment for nitrogen gas.

I am wanting to use copper piping that is oxygen cleaned, sealed with nitrogen – the client states that clean rooms require stainless steel.

Are there documents that can provide guidance?

Your client is absolutely correct, nitrogen should be fed via electropolished stainless steel, sealed in transit to the factory with an inert gas.

Copper is most unsuitable because it will corrode internally and externally.

Many people will also require a specific roughness level inside the tube.

Regards,
Dick Gibbons (committee member of the BSI LB I 30 in the UK and convenes 14644 part 8 and 10 for Chemical contamination)