I'm of 2 minds on this.
If they can prove that she knew he was going to do something anti-social (moreso than just an "improper person" in possession of a firearm) then, yeah, prosecute her for all its worth. But then there are already laws on the books to cover that (conspiracy, aiding & abetting, etc.).
It's kind of like my cousin loaning his car to his brother who has had a license suspension, & then the brother going out & robbing a liquor store with it. Do you prosecute the cousin for armed robbery? Yes, the cousin shouldn't have given up the keys. What he did was wrong (giving his keys to someone he KNOWS had a license suspension) but was he just doing one of those "favours" for his brother, thinking the brother was just running out for a pack of smokes, or did he KNOW (or have a strong suspicion) that the brother was going to knock off the liquor store?
Hard to prove knowledge/intent. And, as with the so-called "hate crimes" legislation, I don't want the authorities wriggling around in someone's head making life-altering decisions based on what they THINK he was thinking & when they THINK he was thinking it.
As at least one other poster said, she's already guilty of a straw purchase. Hitting her with that is probably the best we can do under the circumstances.