Dating a friend's ex?

So today one of my best guy friends broke up with his girlfriend. Then later we made out. I've liked him for a really long time but his ex is my friend. We've never really been that close. Well he really likes me and has been meaning to break up with her for so long but he wants to keep our relationship (if you can even call it that) a secret. What should I do? Some people saw us making out and I told my best friend but I feel bad for his ex. I don't want her to hate me or for people to spread rumours, this was a long time coming and he was single. Did I do anything wrong? Should I tell her or let her find out from someone else? Please no judgements I'm just looking for guidance. Thanks :)