You want to expose them because you're upset and bitter, and rightfully so. Everything you are feeling is perfectly normal and valid... but it is a terrible idea to get involved and cause more drama than there already is. Taking the high road and being as mature as you can, and leaving them to do their own thing is the best move you can make. It will only make you seem immature and dramatic, and it won't make you feel any better. Plus, if she left her fiance, then their relationship probably ended anyway.
I understand you are feeling a lot of hurt, anger, resentment and heartache right now, but you will recover from this. The sooner you collect your belongings and start focusing on your own life, the easier it will be to move forward.
Best of luck to you.
"Caring is not an advantage."