It's the same as any other sort of "prohibited person" situation. Your friend may legally have a gun, but since her husband is prohibited under federal law from possessing a gun or ammunition. Therefore she must take steps to assure that her husband doesn't have access to a gun or ammunition, e. g., lock the gun and ammunition in a safe or other secure container to which her husband does not have the combination or a key.
Just hiding the gun and ammunition or putting it away with her things is insufficient. He must not be able to readily get to the gun or ammunition. See
United States v. Huet, 665 F.3d 588 (3rd Cir., 2012), in which the gun a prohibited person was charged with illegally possessing was not secured against the prohibited person's access, supporting both the prohibited person's conviction for unlawful possession of a gun and the indictment of his cohabitant. From the opinion (at pg. 593, emphasis added):
So the gun Hall, a convicted felon, was indicted for unlawfully possessing, belonged to his cohabitant, Huet. It appears to have been undisputed that Huet could lawfully possess firearms. Nonetheless, she was indicted for aiding and abetting Hall's unlawful possession of gun because Huet's gun wasn't secured against access by Hall.
While Hall was a felon, it doesn't really matter what the disqualifying condition is.