I realize that you have already made up your mind but I'm going to give you some Free Advice a few days early!
***ahem***
There are way too many people advising you to "use her and abuse her... then run away. First, no one deserves to be used. That is a type of pain that never goes away. Second, running away never works... (I'm talking about Carma) besides, running away becomes habit forming.
Ok enough preaching. My advice is call her... have a LONG Conversation (at least 3 minutes ) take it slow. I truly believe that people should not be afraid to roll the dice every once and a while.
And, if after 3 minutes on the phone you are convinced that she is only cheating on her boyfriend to break up with him... Ask her if she has a hot girl friend for a three-way... because HEY FREE THREE-WAY!