Actually d it should be somewhat of a self limiting problem. I would suspect that many of her customers are also attempting to keep live plants in their ponds. If so at some point the salt will begin killing the plants and off they go to seek advice somewhere else. I never heard of putting the vinegar in a pond, but some people use it with calcium hydroxide to supply calcium to reef aquariums.
I want to thank Don for posting this information because it does help to explain the limitations and possible benefits of using salt. For many newbies and people who have learned it wrong, it may a little like closing the barn door after the cows got out, since a simple Google search on "koi and salt" can yield dozens of sites parroting the original koivet position. And to compond the problem some the sites are pretty official sounding.
About all a person can do is look for credible sources of information like Bito here and hope people don't get tired of beginner questions. Believe me I know the frustration of listening to the same silly questions from newbie after newbie. A zen like tolerance is your only hope to maintain your sanity.
Mitch