How do they know to request footage from your cameras? Who told them? What day, what time? Do you really think FAA employees have the time to review hours and hours of security footage?
It's a little thing in our laws called Double Jeopardy. Nothing to do with the game show, I promise. It goes like this: if govco in some form initiates proceedings against you for some infraction and you get off, the same branch of govco can't do it again. They can try and get you for a different infraction, or a different govco entity can go after you. A good example is facing State criminal charges and then having to face Federal Civil Rights charges for the same offense. Yes, it's happened.
Now, if the FAA gets evidence of a second set of offenses, they can ratchet up the bar to primal proceedings provided their case is strong. They have a record of behavior (first offense, as you point out) and now they have evidence he went right along doing it. Photographs of him getting in and spooling up from Ace Detective and photographer extraordinaire Jack Fleetwood (a gumshoe's name if ever I heard it) might be enough. Complaints from pilots and personnel at the airport would help as well. Moreover, if said complaints contain specifics as to timed location, the investigating FAA person can actually review said security footage with some hope of success.
Moreover, if he really is carrying a loaded semiautomatic weapon into an aircraft he may be in violation of more than one state or Federal firearm regulation, since aircraft have a bad habit of crossing state lines. If the FAA won' shut him down the ATF might. They're cops, they have powers of criminal investigation and arrest.
Now I'm going to tell you why you should do this. You can see the guy's a danger both to himself and his passengers, not to mention innocent folks on the ground. How are you going to feel when he crashes and kills and innocent person? You have some skin in this, since you knew it was going on and did nothing but ***** on a message board. It's no different than seeing someone assaulted with the means of stopping it, and you do nothing.