The first thing I would ask myself; "Is this a deal breaker?" In other words do I want to end all contact with this person? If the answer is (no) then I try to imagine myself being in their shoes. (I would only do this if we were really close). It is highly unusal for someone who is "close" to you to "intentionally" do something to hurt you. By close I mean someone you have a track record of having established honesty, trust, loyalty, love, and respect over the years.
There are two basic reasons why people lie:
1. To avoid the pain or repercussions of dealing with the truth. This includes causing someone else pain, dealing with their anger, hurt, and disappointment.
2. To keep the thrill alive. Whatever they are doing behind your back they don't want to stop. Therefore they lie to avoid being pressured to stop.
When someone has lied to us, used us, or betrayed us we are hurt and insulted but most of all we regret allowing them to become close to us in the first place. We start to wonder how many times they've lied to us, were we ever really close in (their eyes). Generally speaking if someone close to me hurts me it will either create some distance between us or I will let them go altogether. We get to (choose) who we spend our time with. I can't think of a reason why I would invest a lot of time and emotion on someone who is not trustworthy. One man's opinion! :-)