It is perfectly on topic.
So say my brother walks into a gun store and purchases a firearm. I call police and tell them that he has the mental capacity of a 13 year old. They can't just go and take his gun away. He is an adult so I can't making him go see a psychologist if he doesn't want to. I guess I could apply for a conservatorship and pay out of pocket to go to court and ask a judge to make me his guardian and that might be a workaround but I am not a lawyer and I am not sure if that would be effective. I might also then be liable for anything he does... needless to say it is not an easy or even obvious solution.
And to clarify, he lives in Washington State.
I have noticed that a lot of the resistance to a red flag laws seems to be based on the worst case scenario. They imagine a situation where someone simply says another is crazy and bam, there goes someone's rights but this is not inherently the case. A properly designed red flag law would require a certain level of proof just as any other police investigation and there would be systems in place to protect a person's rights as well. Imagine a police investigation, they can stop someone based on reasonable suspicion and that is already taking away someone's constitutional rights (temporarily). If police establish probable cause they can arrest a person and can hold them for 48 hours while they investigate further, again someone is losing their constitutional rights.
I think that we could use that same reasoning when applying to red flag laws. Take the person and/or their guns for up to 48 hours upon establishing probable cause. If the investigation doesn't show further proof then release their firearms immediately. Police regularly take away someone's constitutional rights but is has to be based on just cause. Taking away guns based on a third party rumor (for example) would absolutely be an abuse and unconstitutional but we can't assume that every form of red flag law will be that worst case scenario. Again, this is why I say I support it conditionally. I support it on the condition that the revocation of constitutional rights is based upon good evidence, that it is as minimal as necessary and that there are strong protections for the accused. This seems to be the standard when a constitutional right is taken away by police in other areas.
Mental retardation carries a documented and recognizable disqualification for many things, like applying for a drivers license or joining the military. Most people would pick up on his 13-yr-old mentality anyway and refuse the gun sale.
Red flag laws would not PREVENT your brother from buying a gun unless you knew about it beforehand.
You don't understand the term "red flag" if you think evidence is going to be required. in court, a person's statement is considered evidence. If there was tangible evidence, they wouldn't NEED red flags -- the evidence would be sufficient to file a report and arrest him/confiscate.
Red flags are for people who don't have actual evidence, but simply believe (feel) someone is a risk. "I saw EEF screaming at the sky and calling Trump every name in the book. Then I saw him online asking about airline tickets to DC and where to buy body armor. He never threatened to do anything, but I have a feeling he's going to do something violent."
That's a "red flag". It alerts police your behavior is concerning and might indicate violent intent. Nobody needs "probable cause". That's why most red flag laws limit who can report under them: relatives and co-workers, maybe close friends. Even so, anyone can claim to be a friend.
You love to imagine how things should work. As many times as TRO reports are abusively filed, it's naive to think red flag gun laws won't be abused as well.