Because it was her job to ensure it was loaded with blanks. It was her job to maintain the chain of custody. It was her job to know. If someone else messed with the weapon then she failed to do her job.
The guns on sets are to locked up or under the direct control of the armored unless being used for the scene. She, or her assistant were supposed to check the loading, and hand the weapon to the actor personally. The gun was left out on a tray and the assistant director handed Baldwin the weapon. That is not the the way it is supposed to work. I looked t up.
She is as guilty as Baldwin.
I believe the whole of the rule is that Halls and Baldwin were required to check them together. It's called "witness".
Amazing how you can know for a fact that Baldwin did not in fact sneak the rounds into the pistol himself.