I was taught by the RAF how to deal with overconfident students, and I have found that the same can be applied to those in the RHS who need a reallity check. It's quite simple really, and does not in any way involve confrontation, or an "atmosphere" on the flight deck.
Simply set the candidate tasks that you know he will fail to achieve. If you set the goal posts correctly, a post flight debrief will not even be required - he will know he failed to accomplish the desired task, and worse for him (or her) will realise that he demonstrated his inability adequately.