If your Design or Engineering Documents specifically reference Heat Treatment for P-No.s 3,4 & 5 materials in accordance with Paragraph 331.2 & 331.2.1 and state that examination prior to heat treatment for P Nos. 3, 4 & 5 materials is acceptable, then ok. But I doubt it.
If not, and you are performing NDE prior to PWHT for P-No.s 3,4 & 5 materials and not after final Heat Treatment, then yes, you are in direct violation of the code.
Look at B31.3 Section 341.3 "Examination Requirements".
Then go to 341.4.1(b) (1). It requires "Radiographic Examination" for circumferential butt welds under "Other Examinations".
Paragraph 341.3.1 requires these examinations be performed prior to initial operation.
341.3.1(a) requires the Examinations be performed after any heat treatment for P Nos. 3, 4 & 5 materials.
I'm afraid your client is right. You are not complying with ASME B31.3 or normal engineering practices.