I'm wondering how many threads we'll end up with just to explain the differences in HB1815 and HB378 and that they have absolutely nothing to do with each other.
I'm assuming by "castle law" you mean what's been called the "Castle Doctrine" which was HB378. It really has nothing to do with guns or cars...kinda.
HB1815 clarified what is legal as far as carrying a concealed handgun in your car. As long as she meets the points of the law, including not being prohibited from buying or owning a handgun, not committing a crime etc., and she's in a car, in Texas, she can have a handgun, concealed, in the car. This was enacted last fall. The main thing it did was take the "traveling" clause out of the picture because it was way to vague and had to be interpreted and everyone did so differently, or in a way that it worked in their favor.
Of course I'd highly suggest she read the bill (oh wait, it became a law, I'll sing the song about how that happened ) to understand it.