jchi,
You really have two issues here.
First, you, as an engineer, have a duty to protect the public safety and welfare. In this case, you need to firmly, in writing, notify the farmer and the contractor that there is a significant safety issue involved. This is an ethical (the right thing to do) and legal (as an engineer you have specific laws on your behavior here) issue all rolled into one.
This first issue has nothing to do with whether a building code is legally applicable.
You might also seek guidance from the state board as to what you should do under your state's standard of care rules.
Second, in legal, technical terms, if the county doesn't have any jurisdiction in construction, then you may just need to back away from the project as you would eventually be tied into any legal actions if something fell or someone were hurt. I'd hate to do this as I've always tried to resolve things as much as possible.
The owner (the farmer?) would most certainly care if the thing wasn't stable, but maybe depends on the person and how influential the contractor is. Keep hammering away at them and even suggest a second opinion from another engineer.