I gotta say that these threads are a little scary. Here we have somebody who says "A friend is going on a cruise," and we're given no other information.
Then we start giving advice which may or may not be right, depending on information we don't have--in this case, where's the damn cruise going?
Now, there's no doubt that the right answer is always to get a passport. As has been said time and again, it's the surefire, no-questions-asked document that's made for the purpose. And many of the countries that didn't used to absolutely require one are now doing so.
But this is for sure: If "a friend is going on a cruise" and it happens to be a repo to Barcelona, or England, or Amsterdam, I wouldn't be telling anybody to mess around with birth certificates, driver's licenses, voter registrations or Blockbuster movie rental cards. You bloody need a passport for those destinations, end of story.
I don't know why agents keep telling people to fake it with all these inferior forms of ID. I suppose they're afraid that if they mention the need to spend another fifty bucks it will be a dealbreaker. But they really do no one a service in the post 9/11 world by telling them that they can leave the country with anything less than a passport.