I'm happy that my best friend and her boyfriend broke up
Why are you happy that they broke up?
Because she was cheating on him. Somebody had to stop this
She's your best friend, so you shouldn't be happy that their relationship is ending. She may direct her negative emotions towards you if she notices that you're happy about her downfall.
I'm not the bad guy here. Her boyfriend is the confused one.
I understand why you're glad your friend is no longer lying to her boyfriend. You should talk with your best friend about why their behavior was harmful to themselves and to their partner so they don't cheat on other partners in the future.