The human brain has the capability when stressed beyond it's limit to slow down your perception of time to a near stop, this allows you time to react to the situation, you have time to observe your threat and take in the situation.
There is a scale of reaction that occurs with the data you just gained, if it equates the scenario with danger, then the first thing that happens is what is stored in your brain as a reaction to the stressful situation.
The first place your brain will try is what it has stored as reaction to imminent danger; this is in the center of your brain called the control center. Under normal circumstances it is fed information and relays it to the thought center for study, however in an emergency situation it switches control to the disaster center, which bypasses thought, and will engage automatically.
Your heart beats faster as biological chemicals kick in to throw you into overtime, your lungs are operating faster and the fight or flight response kicks in.
You have spent hours holding your arms in a certain position, articulating your hands and fingers to squeeze a trigger that is tied to an action of throwing mass downrange with lethal force, what you may not know is that every time you aim and pull you are reinforcing neural links that were created when you first shot your gun.
Back to yourself in the fight or flight response, your FOFR concludes that your hands are registering the same sensory information that is tied to 'the thing that goes bang and puts holes in other things' and without thought, without reason or consideration, after your control center has enough relevant data to identify the threat and has verified that you are holding a deadly weapon, it will send neurological impulses through the relevant neuron chains and you brain will make your body fire your gun without even bothering to consult you.
Ask any officer who has been involved in that kind of stressful situation, most don't even remember firing their guns, and often their weapons are empty after the encounter.
The OP's friend had never practiced those life saving tactics, never formed the neural links that equated to the life saving action of firing the gun in her hands, without those vital disaster center instinctual reactions to save her live the gun was nothing more than an expensive hunk of metal.
What happened in her mind? It sounds like her flight or fight response failed to come up with a viable response, she couldn't run away and she wasn't strong enough to fight back, so her brain did the only thing it could do, it returned control of function to the thought center. She froze when she realized where she was and what was happening, the BG gave her a reasonable option and her already stressed out mind, still set on survival, reasoned that it was the only available solution and promptly responded by bowing to the command given by the threat.
Conditioned to do so by indoctrination into an idea that she is somehow "morally superior" raped and beaten to someone who has the audacity to defend themselves with deadly force.
Score another one for Hitler's ideology