Maybe it's just me, but I adjust the nav volume up or down all the time depending on highway noise, passenger noise, annoyance factor or my need to know exact instructions. You can adjust volume through the system settings menu, but if I had to scroll through that, I would probably never change the setting.
Just turn the volume control (on the steering wheel or the knob on the radio) up or down while she is talking. The nav volume will change and that will be the new setting until you turn it up or down again. The stereo volume will not have changed. The radio button/knob regulates navigation volume only while she is talking.
Or alternatively, as laurel says, just say "navigation, voice louder" (or softer) and the volume will go up or down one notch. I adjust the volume control manually as I find it easier, but that's only my preference.