Just a reminder and short example. 

Had a great candidate (department level although this is a reminder for everyone) who interviewed with my client's CEO.  The candidate, during the second hour of the 2 hour interview, let his guard down.  The interview was going well, both the client and the candidate felt an immediate match and comfort level.  The candidate was so comfortable that when asked directly about his current company's CEO, the candidate made some honest, but less than flattering statements about him.  When asked by other company execs if anything during the interview(s) concerned him, he said he was a bit concerned with the frankness and negativeness the candidate spoke about his current CEO.  He did hire the candidate, but keep this in mind......no matter how comfortable you are during an interview, better to not speak negatively about your company or staff there!

As a Third Party recruiter, this is where our coaching role comes in. CE Hunter, using your situation as an example, I assume that as a his recruiter you were aware of these real & honest issues as they were likely his main motivating factors for even exploring anything new.  Knowing this type of information, which you indeed should, it is your responsibility to prep the candidate and coach him to prepare him for that particular line of questioning.  Under no circumstances should anyone trash their current employer, no matter how bad the situation.  One can easily get their point across in a much more mild mannered way with a little coaching.