There is no magic pill here. We've had this discussion on the forum several times before.
Leaving aside any criminal trespass implications...
You have to decide whether losing the job is worth the risk of trying to change the policy.
As it is now, if nobody knows she carries, as long as that is the case, no harm, no foul. If you start asking questions, well... they are going to start asking questions too, as in, "are you carrying a gun?"
If you aren't prepared to find a new job, my personal suggestion would be to not bring it up at all.