If this really is a 712, then it is a machine gun according to the definitions of Title II of the National Firearms Act of 1934.
If that weapon was registered as a legal machine gun when imported, or during the very brief machine gun amnesty period in 1968, then it can be transferred to a new owner on a "Form 4" or to an heir on a "Form 5."
If it was not registered, then it CANNOT be registered. It is illegal contraband and its possession can mean 10 years in jail and a $250,000 fine. (As well as the loss of all firearms rights, forever, if you do manage to put your life back together after 10 years in the "pokey" and the loss of a quarter of a million dollars.)
Probably the safest route is to have an attorney contact the ATF and investigate how the weapon may be surrendered without incurring the legal penalties. Or, wait until the uncle's passing and have the estate's executor clear the matter up (surrender the weapon). Whatever your friend does, he should not ever take possession of it. That would be a "no-fooling, life changed forever" kind of mistake.