Hi Lisa,
The level of oversight and regulation depends on whose herbs you're using. For example, if you use Jing Tang products, he's quite right, since the FDA hasn't screened those products and since their manufacturing facility is not GMP certified, as far as I'm aware. Someone in the know, please correct us if we're wrong.
In contrast, Natural Path products are made in GMP certified facilities all the way down the line, and are tested on an ongoing basis by the FDA, the CFIA (Canada) and the AQIS (Australia).
So if you're using our products, then his comments are in error. In fact, if he hasn't checked his facts, they are unscientific.
In terms of research, generalizations are once again inappropriate, since herbal formulas and their constituents may, in the case of one product, have never been tested, and on the other hand, be the subject of 50 or 100 studies on pubmed.
Perhaps the appropriate way to handle it is to voice your concern with your state regulatory body, since he is indirectly and unjustifiably besmirching your reputation. In Canada, this would be grounds for disciplinary action. Before you proceeded with that, though, I would just write a concise letter stating the above and asking him to desist with his derogatory comments. Also give him the benefit of the doubt and say that this is the impression clients have of him, but if he is guilty of these claims, he should stop. You could also let him know your next stop would be launching a complaint with your state or provincial licensing body, but that you really don't want to do that, and are only asking for ambivalence, and the same faith in you veterinary school training and sense of professionalism that you place in his. Unprofessional comments like the ones he's making reflect as badly on him as on you, when they are so sweeping as to prove unlikely to be true upon even cursory examination. You may want to bring that to his attention as well.
Hope that helps,
Steve