I'm not going to my friend's wedding.
Why would you not attend?
I'm not happy and I think it will end in divorce in the next ten years.
Your friend's marriage truly might end in divorce, but they might know something you do not. You should attend and be supportive and if your prediction is correct the you should also be supportive.
I don't give a shit about him and think he is wrong for her.
You can't be expected to like who your friends get in relationships with but you are expected to be there for your friend. By not being there in her special event, you are saying that it doesn't matter to you.