When at James' barn, before going to him, you can talk to AJ and he'll question what a salt lick is (as its in front of him), doesn't matter if you tell him to lick it or not, the achievement will still unlock.
Though if you want a little reference to Season 1 Episode 2 then tell him to not lick it and AJ will question how do you know with Clem replying "I don't know" :)