So is it safe to remove the brace and throw it on the shelf if you have a registered SBR that you can possibly use it on? Asking for a friend….
I suspect that in reality it'll depend on how pissed the gov is at your friend.
If they kick in the door for selling machine gun parts online, or a metal card with a picture on it, and find a pistol brace, and an unregistered pistol that could accept it, I'd bet $20 and a bottle of bourbon they tack a constructive possession charge on the whole thing, regardless of any SBR lowers in the house.
I doubt strongly that they are going door to door with SB Tactical's customer list and checking to make sure everyone cut up their pistol brace.
Barring some
other behavior to draw the Eye of Sauron to your house, I'd be pretty surprised if a pistol brace, unmounted, in a closet somewhere got anyone in to trouble with the feds.
If I were to Red Team this, and I was a shitty Revenuer, I would have my guys watching rumble vids and cruising Reddit for someone that I could pin a white supremacy/proud boys/creepy porn tag on who is also talking a big non compliance game online. A flashy tac team raid on some scary domestic terrorists with dangerous unregistered SBRs is exactly what is needed to get the normies riled up and on the cop's side. Raiding a suburban dad who bought a short AR in 2020 at Cabela's or Bass Pro and capping the family labradoodle isn't going to have the same punch on Twitter.