They can't legally fire her for having a gun concealed in her locked car in the parking lot unless they meet one of the quite limited exceptions in TX law.
HOWEVER, the burden of proof would rest on you to prove that's why she was fired and that could be very difficult unless they were stupid enough to tell her why she was fired in front of witnesses or if they gave it to her in writing. Otherwise it wouldn't be easy or cheap.
As pointed out, TX is a right to work state which means they don't have to tell you much of anything to fire you..
Second, there's no penalty in the law for failing to comply with the law. Which means, at best, she might be able to get the job back and maybe some back pay if you could prove why she was fired.
Read the law and make sure that she's compliant with the law. Then determine the risk of being caught and being fired to the best of your ability and decide whether you're willing to run the risk. If they've never searched vehicles, it's highly unlikely that they'll start now--especially with the passage of the new law last year that would make open enforcement of a no guns in the cars in parking lots extremely stupid.
At the same time, work with TSRA and my state senator to force companies to follow the law.
Be circumspect. It's not uncommon for companies to consider their policy documents to be proprietary.
For example, if I were to provide a third party with policy documents from my company I could be terminated for that offense if they determined I was the culprit.
The point being, don't get fired for something you do in the process of trying to keep someone from firing you.