Do I really have to go to my best friends wedding?
Why would you skip such an important event?
Because, I don't believe she'll be happy with him. I'll inform her about my decision later.
I get that you may not like their spouse, but it is your best friend.  It's expected that you will be there for her on her wedding day, and it would be really hurtful for you not to go.
So I should go there to be the best man for them?
It is expected to be there to support your best friend on an important day. Even though you don't think the marriage might not work out, you should just be there for them.