CURRENT NY State “kosher laws” are really “labeling” laws. Anyone who offers certification must fill out a form that among other thing details (in the words of NY State) “a statement of their qualifications for certifying food as kosher.” The state of NY will not get involved in determining if the shulchan aruch and nosei keilim or lihavdil the jts handbook on kashrus is what determines kosher. You can definitely say this persons qualifications do not meet my standards, but you can not say “the food that person certifies is not kosher”. The more private an individual the better chance they have at a successful defamation suit.
