Did I tell you the story of when I gave my friends boyfriend a hand job?
No, and why would you tell me that.
I just think it's a funny story.
Wanting to tell me a funny story is fine but you're telling my about how you got your friends boyfriend to cheat on her with you. Did you even consider her a friend because that's an extremely wrong and underhanded thing to do. I assume she doesn't know because if she did you wouldn;t be friends anymore.
I did it because I wanted to get laid.
You were being selfish and not a good friend then. You should tell her the truth.