Use the fill flash

When you're photographing a person in front of a snow scene on a sunny day the lightness of the snow in the background of the shot will fool the camera's automatic sensors into exposing incorrectly for the person who is standing in front of you. In a similar manner to taking photographs on the beach in the height of summer use the camera's fill or force flash when shooting in these circumstances. The flash will light the person in the foreground and you'll have both the person and their background nicely exposed.