How should I confront my Mom?

About 2 years ago a family friend told me that my mother started smoking. when i asked her about it, she told me she would never do that(my family is strongly against smoking) and said that she had tried once when she was 20 but hated it. every month or so she comes home smelling like cigs alot, she blames it on her friends. Today, a friend of mine saw her downtown smoking a cig, and asked her about it- she said not to tell me, but of course he did because he was my friend... im 16, my mom is 51. how should I confront her? she has always asked me to be truthful, to be honest, and how can I trust her if she is hypocritical like this?