She owns one gun but actually owns 7. Her parents tell her to sell it. She's 28, an adult and her parents have no hold on her.
She is under mental help which her Doctor would know that she was dangerous. But, again, she's 28 and an adult.
So, with a good solid Red Flag Law, the Parents would not only suggest that she sell the one gun they knew about but get with the shrink and a judge to secure whatever other weapons she might have access to until it can be determined that the dangers have passed.
You keep making excuses and they keep murdering en mass and all are using the same weapon. If you won't take the weapon away one way, take it away another way. Otherwise, you may just as well be reloading that AR for them so they can get even more children murdered.