I'm going to tell my friend our mutual ex is trans
How can you leak out such a secret about your ex?
Becuase I think it's important to be honest with my friends.
It is not being honest to divulge someone else's personal information. It is not your right to divulge someone else's secrets. If you want to be honest with your friends, tell them something about yourself, not someone else.
I don't care if I hurt someone. I'm going to tell them anyways.
I really think the best action is to just keep it between you and your ex. When your ex is ready to tell others, they will. Don't think they'd feel even more betrayed that something so important was leaked out?