I'm going to sleep with a friend's ex without telling him
What is you reason for doing something like this?
I want him to be hurt and I want the pain to last.
If you want to sleep with her you should be honest with your friend. Wanting to hurt your friend is not the way to go.
It's complicated.
Just know that going through with something like this can be the end of your friendship. Just understand the risk before making your final decision.