I'm going after a girl my friend liked even though she liked me and he says that about everyone
Why do you want to go after a girl your friend liked?
I'm just confused I thought they liked the girl I'm going after
You should definitely talk to them about it, explain the situation, and ask them what they think. You may have to learn to accept their decision and try to support your friend.
I don't care. I'd do anything to make her mine.
Friends are expected to be respectful of one another and not go behind someone's back. If your friend finds out you went after a girl he liked without talking to him first he will be upset. It's better to talk this through before betraying their trust.