Applicable references: DAOD 2002-6 CEOTP; CFAO 49-4 - Career Policy NCMs Reg F; and CFAO 11-6 - Commissioning and Promotion Policy - Reg F.
After a quick look over those three refs, I saw nothing that directly applies to this situation (NCM to officer). However, both CFAOs state, essentailly, that if you change occupations you are promotable until the date you change occupations - see CFAO 49-4 paras 26-30 and CFAO 11-6 paras 29-31.
While this situation is somewhat different, the overarching policy intent would seem to be "You're promotable until the day you take on your new occupation.". In this case, I'd suggest the individual inquire (in writing) to the career manager why the promotion message is being rescinded. The policy as written says he remains promotable until 17 July. That he'll only spend two weeks in that rank is irrelevant.
Besides, this way he'll be buying drinks to get out of the Jrs mess, into the Sgts mess, and then out of the Sgts mess and into the officers mess, thus supporting NPF.