The gun in question is ALREADY an unregistered NFA item.
I would suggest that you run away from this deal and encourage the owner to voluntarily turn it in to the ATF for destruction.
And that is actually the very best advice.
Technically, if this is now an unregistered NFA Title II weapon then it is now, and forever will be, contraband. The only lawful recourse granted by the BATFE is to surrender the weapon to them (preferably through your lawyer!).
If all you own is one barrel, and all someone else owns is one barrel-less receiver, it becomes a bit of a "tree falling in the forest" kind of situation, as the
de facto SBS does not physically exist.
I've never heard of someone being prosecuted for taking possession of a firearm that they had reason to believe
had once been configured as an unregistered NFA item (so is technically now once-and-forever illegal), but I sure wouldn't want to have to fight that charge, either.
Truth be told, I'd personally have no compelling desire to have any involvement whatsoever with such a thing.