It's possible she was too appalled / stunned to make a sound.
I mentioned up thread that I have fired a pistol in an indoor shooting range. There was carpet on the floor which helped to soften the sound, and the room was huge. I wore ear defenders and it was still very, very loud.
OP's bathroom was small and fully tiled so the acoustics would have been loud, hard and echoey. The noise of the gun going off would have been tremendous. His ears would most certainly have been ringing. If she made a sound he wouldn't have heard.