There is a fader, but it's for the stereo, not the intercom. Push the audio mode button until FA comes up and then you can adjust your speakers. Intercom volume is controled through the VA Mode button. Push button until the headset indicator shows, then adjust the volume.
Try switching the helmets so that you're connected to the back plug and she to the front and see if there is a difference. I found the problem was always in the headsets and this will tell you if her headset has a bad microphone.
Personally, I like it when my wife can hear me but I can't hear her.

RainMaker