I slept with my ex's close friend
That's a big yikes. Why would you sleep with her?
Because I was drunk and sexually aroused
You should not try to make conflict in other peoples relationships. You should not have anything to do with you ex and drama.
But it's not like they will be friends forever. I always make sure that my ex's friends would want to be with me.
Don't you want your ex to stay as an ex? Being involved with their lives still is a recipe for disaster, especially sleeping with their friends. How about finally cutting them off for good, and seeking the happiness you deserve without them to pull you down?