There is nothing in the law or regulations I have seen that requires any sort of proof that the rifle was moved out of state.
If you have somewhere to move it, just do so. If they hassle you, just send them a nice letter explaining that you drove it to your grandfather's house in new Mexico or wherever and let them worry about proving it. After all, how do they really know you had it in the first place? His notification is not proof of anything without the rifle.
If they want to hassle you over it, tell them to get a warrant and come look for it. If you moved it, they will find nothing. I have a hard time believing that a judge would issue a search warrant on the basis of some letter or phone call anyway, but either way, if they do come knocking and find nothing, they have no proof of any illegal possesion of a so-called "aw".
Case closed unless they can convince someone to file criminal charges on the basis of some letter. Not likely.
Lastly, depending upon when your friend actually bought the rifle, along with some other legal requirements, the first offense is only an infraction and, believe it or not, a judge could give it back after a hearing (fat chance!).
Ray