No, but presumably he bought it becuase he's trying to finish an incomplete gun, but that's just a creative assumption on my part. If he's already FA-10ed the gun (because it was previously assembled as such somehow) then its not required at all.
He would only have to FA-10 the gun if adding the upper was the last step to making it functional, AND if the lower wasn't already registered with the state.
If it was already a functional gun registered in the Commonwealth, and this is just a second (or more) upper being added, then NO action is required of the OP other than to enjoy his new purchase.
No, but presumably he bought it becuase he's trying to finish an incomplete gun, but that's just a creative assumption on my part. If he's already FA-10ed the gun (because it was previously assembled as such somehow) then its not required at all.