But I am a bit confused. You have a 9" barrel upper, and you have a lower that it fits, so you already have an SBR and it is not registered. You should have obtained Form 1 approval before buying the upper. I hope I am wrong, but it looks like you have already violated the law.
He could be in violation provided the lower was a compete lower configured as a rifle in the same location as his 9-inch upper.
But if it's a stripped lower without anything else (also notice he said "new, never been a rifle") it can't really an SBR as you could assemble it into a pistol. Also they'd have to be in the same location. If you own an under-16 inch upper, but it's at friend's house while your lower is at your house, it's kinda hard to prove "constructive possession".
I wouldn't be strolling around a gunshow with it baiting the bear so to say, but I doubt the black helicopters will descend upon your place for having a 9-inch upper and a stripped lower. Unless you have a bunch of other unregistered lowers laying around.