I'm going to tell my friend's parents about her relationship as revenge
Why would you tell them about the relationship?
She didn't give a shit when she told my parents about my relationship. Why should I hide things from her parents
Neither of you should expose your friend's secrets. Two wrongs don't make a right.
That makes no sense, aren't you supposed to fight fire with fire?
Don't you want to be someone your friends admire and look up too rather than someone who ruins their friends relationships? You could potentially ruin not just the relationship with their partner, but also with their parents. The only secrets that should be exposed are ones that would be physically or mentally dangerous for them.