She ought to have known that purchasing a gun for someone else is illegal. However, true to liberal tenets she was not charged.
It was a rifle, suitable for sniping, IIRC. However she did not break the law. Buying a gun as a bona fide gift is legal, as long as the person you give it to is not prohibited person. There are details, but in general that's the deal.
If you buy the gun with someone elses money, that's a straw purchase, and illegal. If you buy it with the intent to sell it to them, that too is probably illegal (dealing without a license) and certainly is if they are a prohibited person. AFAIK her son is/was not a prohibited person. She *should* be, but isn't.