Glad to help. Two more observations:
The ordinance regarding poop's intent is clearly intended to make pet owners clean up poop off public property and other people's private property that isn't yours (for instance, picking up as you walk down a sidewalk through a neighborhood full of houses). This isn't that kind of case if you have permission from the owner for the purpose of training dogs. The only person who could or should be able to make a complaint is your owner, which he hasn't and probably doesn't care if its a field and not his manicured front yard. Take several photos to show this is not another citizen's front yard. In addition, there is no other neighbor who could make the same complaint on another's property because they would be tresspassing before the poop could ever affect them.
Finally, if the animal control guy doesn't witness any offending behavior with his own eyes, he can't testify to what anyone tells him because it is inadmissible hearsay without an eyewitness. Case closed. The best way to either end this or find out who your nosy neighbor is to accept a citation that requires the complaining witness to appear or the ticket is dismissed. If you go once or twice and the witness never shows, the court will let the animal control officer know they don't have to keep issuing tickets for someone who won't show up and see it through.
Go get 'em!!