Hmm.. that is a toughie.
But yeah, he is going because he probably wants to party in the DR and chase after women in bikinis... And how is she going to handle him oogling other women, and possibly have something happen down there?
He isn't going to give a rat's @ss that it's your wedding.
And if (more than likely when) the drama starts up... who do you think is going to be dragged into it? And even moreso, say you don't "help" her, what happens then?
Do you really need the possibility of crying, the yelling, the name calling going on between them in front of you and your family?