Dealing With A Lie

Has somebody ever told you a lie that was so large it changed the whole way you thought about them? How did you handle it?

I recently found out somebody I care quite a bit about has been lying to me about something pretty big. Without giving too much away, this lie was something I can't just forgive and move on with. The only problem is - the person has no idea I found out they're lying. So now me dilemma is: Do I tell them I know they've been lying, or do I just stop contacting them and move on with my life?

What would you do? Would you confront them or just ignore them and move on? Or do you have a better solution?