If that's the site's policy and they consistently adhere to it, then why is it the site's problem? Sure it sucks for whomever was defamed but that goes back on the person who posted the defamatory statements in the first place. It's worth pointing out that this particular aspect of the case only concerns the forum in which the statement was made. The person who makes the defamatory post is 100% responsible for what they said. The fact that the defendant knowingly posted a false statement to a website that makes it perfectly clear that they will not remove the comment should be a factor in determining the level of damages owed to the defamed party.
In your friend's case, she should pursue Bridezilla for the damage caused to her business. Saying things on a public forum has consequences. I've never actually posted anything on RipOff Report but I did go through the process once just to see how it works. They make it abundantly clear to the poster that they are completely responsible for anything they say.