Why tell him? What does it hurt for him to believe what he believes? It’s not like you working in a museum requires that you tell everyone they own fakes.

YOU DON'T tell him. 1) she is your gf, not your wife. which means it could cause some bad blood. 2) Fell free to discreetly tell your gf and let her broach the topic in a wise time/place. Unless you are set to inherit the sword, I'd recommend keeping your mouth shut. you could avoid the topic by saying that you don't do free consulting on historical artifacts. Also encourage him to do some DNA testing to better tie the sword to a specific family. That may teach him some stuff all by itself.

Gently. Very gently. He's the one with the sword.

Just let him enjoy the story. You should know this is how History works. I own a 19th century Japanese Tanto. I don't know if it's bone or ivory, if it's just late Victorian tourist tat or actually genuine. It belonged to my Dad. He collected all sorts from round the world in the merchant navy and the RAF. I've never had it appraised because I'd rather enjoy the possibility that it *might* be an actual Treasure. We're Human. Treasure is one of the things we love best of all. Let him and his family love theirs, it harms no one and when it gets too much for you, just go to work and fondle some REAL Treasures.

You want to piss off a guy with a sword? You're not very bright, are you?

Encourage him to go on Antiques Road Show.🤣

no need to tell him. if he ever finds out on his own then so be it. no need for you to burst his bubble.

Gotta agree with everyone else on this one, don't tell him, unless he asks. If it's something he's proudly repeating without having made any attempt to verify it, that's probably not information he wants. Let people have their delusions, unless they're causing significant harm to themselves or others.