Since the CPL includes all PPL privileges, a PPL FE conducting tests for a rating attached to a CPL (which might equally be attached to a PPL) is perfectly entitled to do so and to receive remuneration.
I recall this coming up at an EASA meeting - I think it was Germany which had a different interpretation? They were asked if the CPL holder regraded to PPL, then applied for test, obtained a pass for the rating and immediately regraded back to CPL whether they would accept that rating being included in the CPL - and if not, why not. They recognised the futility of such an administrative workaround and fell into line with those who held more sensible views!