He had a PA-15 milled lower from Palmetto Arms. Bet you soup to nuts that Palmetto Arms gets sued and this becomes justification for banning ghost guns.
A stripped lower is still a firearm, and not a ghost gun.
His lower had a serial number. It required a Form 4473 to purchase. He obviously lied about the involuntary mental health commitment on the 4473. Palmetto has no obligation to do more than check the 4473 for selection of any disqualifying information. For some purchases, a NICS background check is done. Again, a NICS check is only as good as the underlying database. If the mental health confinement was not reported/recorded in the NICS database, there was no indication of a problem to the seller.
The "ghost gun" label applies to 80% complete lowers that have no serial number. Additional work is required to make them functional. There is no "ghost gun" argument to be made here. It was a simple purchase of a serialized firearm with undetected perjury on the part of the buyer.
I have a PA-15. Why would you call it a ghost gun?