I'm thinking about telling my friend that his friend wanted to go out with him
Why do you feel the need to tell a secret like this?
I just get bored when his friend comes over to me, he's so stupid.
Sometimes it's good to try to be a wingman, but it is still not your place to tell others' secrets. They could end up getting embarrassed, and it is their feelings that you need to consider, not yours.
I'm fine, I'm an asshole.
Just remember that this is your friend that you are talking about.  You should be considerate of his feelings, and of other people who wanted you to keep certain things secret.  This other friend might no longer be interested in your friend, and you are just creating an awkward situation for everyone.