Here's a longshot, but how sure are you that she can't carry a gun?
Fairly certain the law states a "child" (under 17) cannot have access except under certain circumstances.
It's legal to carry in your vehicle if it's concealed, you're not a gang member, not committing a crime, and you are legally allowed to possess it.
It also states that it's legal to carry a gun if you're in charge of your business location, ie. closing by yourself. I do not see any age restrictions (aside from the aforementioned 17+ requirement).
I'm not a lawyer but unless I'm missing something it would be legal for a 17 year old to vehicle carry and bring it inside when others leave and she is put in charge of the location.
If I'm dead wrong someone fill me in.
Editing to add relevant statutes. Does fed law regulate possession age?
Sec. 46.02. UNLAWFUL CARRYING WEAPONS. (a) A person commits an offense if the person intentionally, knowingly, or recklessly carries on or about his or her person a handgun, illegal knife, or club if the person is not:
(1) on the person's own premises or premises under the person's control; or
(2) inside of or directly en route to a motor vehicle or watercraft that is owned by the person or under the person's control
Sec. 46.13. MAKING A FIREARM ACCESSIBLE TO A CHILD. (a) In this section:
(1) "Child" means a person younger than 17 years of age.
(2) "Readily dischargeable firearm" means a firearm that is loaded with ammunition, whether or not a round is in the chamber.
(3) "Secure" means to take steps that a reasonable person would take to prevent the access to a readily dischargeable firearm by a child, including but not limited to placing a firearm in a locked container or temporarily rendering the firearm inoperable by a trigger lock or other means.
(b) A person commits an offense if a child gains access to a readily dischargeable firearm and the person with criminal negligence:
(1) failed to secure the firearm; or
(2) left the firearm in a place to which the person knew or should have known the child would gain access.