Why would she not want the gun back?
The police probably found her thru the sale form trail from manufacturer to distributor to retailer and then to the individual sale. They have done her a huge favor. It's only rarely that they bother to attempt to return it.
I would say that having a recipt for return would be a pretty fair thing to have. She's done nothing wrong, and if they are returning it you can be pretty sure that the police have no further interest in retaining it "now or in the future".
If your car was stolen and then returned, would you refuse it?