Where can I get information to protect someone against false accusations?

There are two small Aircraft Maintenance shops within a few counties of each other. Our friend worked at shop #1, until they lost their customer because the own put his own plane before his customers. No customers no work. So our friend went to shop #2, and so did some of shop #1's customers.

Shortly after our friend started working at shop #2, a friend of the owner of shop#1 came to a Fly-In Breakfast at shop #2, and he flipped his plane. He did not want his friend at shop#1 to find out, so he had shop #1 fix the aircraft.

The man was not happy with the cost, nor the wait it took to get the new parts that were on order. He claimed that he wished he had not bought the plane. That he resented ever purchasing it. (Buyers Remorse). After the plane was repaired by our friend who has an A/P license, it then had to pass an I.A inspection, and then an F.A.A inspector who had passed it as Air Worthy.

The man would constantly call shop #2 with complaints that the plane was not working right, and that he had taken it to other shops who said it was not repaired properly. The owner of shop #2 kept telling the unsatisfied customer to bring the plane back to the shop to be reinspected and repaired (free) if needed, But the man never brought the plane back, nor did he pay his bill to shop #2.

About (give or take) a year or so later, the man took it to his friend at shop #1, by this time they also found out were their past customers went (to shop #2).

The next thing we hear is a complaint charges were filled with the F.A.A against shop #2.

And slanderous rumors were being spread through shop #1 about the incompetence of the A/P Mechanic.

We then find out that the F.A.A inspector has a long past vendetta against the I.A inspector, and our friends license, and lively hood caught in the middle, and is being threatened if the F.A.A cannot get the goods on the I.A.

Correction shop #2 fixed the man's plane, not shop #1.