This doesn't change the fact that she has to talk out loud. I don't like to take personal calls at work, either. I usually go out into the hall or somewhere more private. I don't think this is rare behavior.
It doesn't talk out loud if you hold the phone up to your ear. To use Siri with minimal noise (aside from your own speech):
Push the power button to turn on the screen.
Hold the phone to your ear.
Wait for the be-beep sound, played quietly through the ears-only speaker.
Speak.
Keep the phone against your ear. Siri will respond through the ears-only speaker.
