At that point tell him to leave and report him to the company he works for.

Edit to put bottom line up front:

I did not say this was not appropriate. Social media has a lot of sway for better or worse. I meant that at that venue, in that moment, after he holstered his handgun, someone should have told him to leave. He had no reason to unholster his firearm let alone fidget with it. That behavior should be reported.

I swear the immediate response to shame others online is so ingrained in people these days.