Usually the lower receiver (officially the "firearm") is listed as "Other", leaving you free to build it into a pistol or rifle.
Per BATF, if you build it as a rifle first, it must remain as a rifle. If you build it into a pistol, you can convert back and forth.
Once the lower is registered as "other" or pistol, that's as far as MD goes. I don't think there's an issue with swapping calibers on a registered lower.
BGOS* is endemic in Maryland.
*Battered Gun Owner Syndrome. It refers to over-scrupulous paranoia regarding any firearm in the "Free State".
AFAIK, the gun was purchased as a pistol. It was bought whole.
How it was transferred should have no bearing. You can change the barrel/caliber of any handgun as long as the design permits. He should also be able to build it into a rifle or SBR as long as he has the proper paperwork(for the SBR) if he so desired.