I don't see why a prompt from the PNF to the PF on any aspect of their functions would be objectionable. After all, they're operating a two crew aircraft together as a team
But we are talking about a government regulatory test of one pilot's instrument flying ability. It is not a crew operating on line and it is not training - it is like any examination - it is a test. Is prompting allowed in an examination? Of course not. While the support pilot should operate ancilliary controls at the request of the pilot under test then all decisions should be made by the applicant for the renewal or issue of the instrument rating.
The ground theory component of the test is a question and answer session. While both the applicant and his support pilot may be together during the session, the support pilot certainly would not be permitted to prompt the applicant during this part of the test.