Isnt threatening someone with a gun already illegal? So no red flag needed.
The investigators would need to talk to whomever was actually threatened.
This is definitely a "red flag" situation, in that RF laws allow the OP, who wasn't threatened, to file a police report -- assuming the law is written as such. If the gun's not registered, and the Cops can find it, that alone removes that specific gun from the situation.
So, we have:
(1) A guy with an illegal/unregistered gun he illegally carries & threatened "people".
(2) A "family member" knows about the threats, either from direct observation (witness), direct involvement (was the threatened party), or someone told him/her (hearsay), and
(3) A "friend of the family member" (OP) wants to know if someone (Family member? Himself?) can report this threatening person to the Cops anonymously to avoid retaliation.
When it comes to reporting criminal behavior, you'll need evidence or direct involvement. The Cops will go talk to the gun guy, but unless he admits what he is accused of doing (threatened someone with a gun), I doubt they can do anything. After the Cops visit, he'll absolutely know someone reported him. He's liable to go off on the first person he thinks could have called the Cops -- maybe the person he threatened? That would make sense.
So, calling-in an anonymous report to HPD could escalate the situation, causing the gun guy to retaliate against the wrong person.
The best solution I think is for the gun to be taken away, either by family or the Cops. Does gun guy live with anyone who'd be willing to confiscate the gun while he's asleep? It can "go missing", no Cops are involved ruining anyone's life or extinguishing it, and he might just think he left it somewhere and can't remember.
Sure, he can always replace it. That takes time and money. It might keep him from being a danger to others for awhile.