A complete stranger trying to take the weapon would be more realistic then a friend or trusted neighbor trying to take it.
But, to put a fine point on it, this isn't Mags' problem. If his friend agrees to keep a locked safe that he doesn't have the key to, and at some point down the road he decides to break into it and take illegal possession of the gun, Mags'
friend is completely at fault and liable to be prosecuted. If Mags has the keys and no evidence can be produced to indicate that the buddy was encouraged or enabled by Mags to take active possession, I fail to see where the risk lies.
If I was planning this kind of thing, I figure I'd only do so with someone who's character I respected enough to trust that they wouldn't do such an illegal and unethical thing.
Of course, as the ATF has clearly said, if he removed the short barreled upper (maybe stick it in a safe deposit box, or just sell it), he could give or loan, (even sell, etc.) the receiver to his pal with no problems whatsoever. If his buddy wanted to put a 16"+ barrel on it, he could shoot it every day without running afoul of the law. (As long as the buddy did not also possess a short barreled upper, himself.) Then Mags could return, retrieve his (or buy a new) short-barrel, and install that without having to re-register the weapon.
About half of the ATF's Short-Barreled Rifle FAQ is devoted to this type of question:
http://www.atf.gov/firearms/faq/national-firearms-act-short-barreled-rifles-shotguns.html
It doesn't have to be removed from the registry, and doesn't have to be re-registered, and while it is not in SBR configuration, his buddy can be in legal posession of it for all lawful purposes -- "win--win!"