Deep snow is ok, till it gets to point where you dont have clearance left anymore..So dont be much afraid of it, its damn icy/snowy polished road where all bets are off .

Just make sure to tell her that 4 wheels are only good to get you going, but will not help to do any stopping (except for engine braking of course). So cornering is something to pay lots of attention to. I know it sounds bleeding obvious, but there are lots of occasions when people aint getting it seriously and ending up in ditch/rolled/falling out from road. Also it helps to train on how to brake with engine on slippery road.