Have her place her face underwater while breathing from the regulator. No mask. Nose must be underwater to do this. A natural reaction would be not to breathe from nose.
The issue is that she is a
nose breather... which actually means that her 'natural reaction'
is to breathe from the nose.
It is normally under these circumstances (no mask breath, mask remove) on a scuba course that 'nose breathers' suck up water and start to hate the course.
I am sure that is what the OP is asking for advice about
I've seen this many times when teaching scuba. It's more much common among women than men. It takes a little time and patience to overcome. The tendancy itself can be overcome with practice - but initially it requires a 'work around' so that the diver concerned doesn't have too many unpleasant experiences with water up the nose. Unpleasant experiences can discourage the student from continueing training.
I know that some instructors might disagree with me, but I am an advocate of 'nose pinching' for people who suffer from this. Basically... anytime the mask is removed, then just immediately pinch the nose... the breathing reaction will automatically switch to the mouth
and water cannot enter a sealed/pinched nose.
This isn't ideal as a permenant solution, but I have always found that the person quickly adapts to the routine of mouth breathing underwater...and the nose pinching swiftly ceases to be necessary.