I think I ruined a good friend's future relationship.
Why don't you talk to him about it?
I'm afraid of telling him how much I hate him for not telling me anything about his plans to be with that girl.
Even if you feel some type of way about your friends relationship you should do your best to stay out of it. You can talk to your friend about it, but don't interfere as this can break trust.
I am not a good friend. I don't trust him and I don't support his relationship.
All you can do is tell him briefly about your concerns, and then you have to leave his relationship alone. It's not your business, and he'll resent you if you interfere.