Regardless of who has a gun, neither one of them have a right to bring a gun, wave it around to threaten each other, or shoot at each other.  We're not a vigilante state.  Want to practice that type of behavior, go right ahead and you'll wind up in jail and hopefully for a long time especially if you wound or kill someone.
You have a right to CALL APD and make a report that someone has a gun and is making a threat.