A pattern of lying is a big red flag and ultimately will destroy the relationship. People who habitually lie often dont ever break the habit. No body wants to be with someone they can't trust to be being honest with them. I would sit down hand have a heart to heart and be very clear about the lying stopping here and now and if she continues to do so I would walk away from the relationship.
Read:Intellectual Foreplay by Eve Hogan, and Steve Hogan