I'm probably in the minority here, but it's Josh's wedding day too. Yeah, I know he probably hasn't dreamed about it like girls do, but it's still an important day for him also.

If he wants her there, she should be there. If she doesn't want to be there, problem solved. If you guys are old enough to get married, you're old enough to work this out. You guys don't have to be best friends but Josh will be a lot happier if you guys get along. You both must have something in common if you both love Josh and he loves both of you.