Please tell your friend to contact the Federal Bureau of Alcohol, Tobacco, Firearms and Explosives (BATFE) immediately, or to consult an attorney. That is a Russian PPSh submachine gun, or a Chinese copy. Either way, it is illegal to own unless registered. It is possible that her husband registered the gun in the 1968 amnesty, in which case she needs to take steps to get the ownership transferred so she can sell it if she wants. It would be worth a considerable amount of money.
If it is not registered (and BATFE can tell her), it is illegal and she will have to surrender it to the government. There is no legal way she can keep it, none. If she feels uncomfortable dealing with federal agents, she can have an attorney handle the situation. There is zero chance she will be prosecuted or even inconvenienced, but she does need to properly and legally dispose of that gun; it is contraband and she is taking a risk if she knows about it and chooses to do nothing.
Jim