I would definitely be understanding, especially if I changed the date of the wedding. Friends are forever, in my book. Now about her cruise. My friend got married on a cruise, but it was just him and his wife. The wedding party didn't go on the cruise. The boat stayed docked until after the ceremony. Wedding party got off the boat, newlyweds stayed on. Maybe your friend's could be the same way? That way, there could be a slight possibility that you could still go, if your calendar (and pocket) permits. Just a thought!
