I do not believe he is worse off; if anything his chances of obtaining a not guilty verdict have increased significantly. When witnesses/victims change their stories, it tarnishes their credibility. In criminal trials, the State has the burden of proving the Defendant guilty beyond a reasonable doubt. I can not fathom how a Judge or Jury can convict based on unreliable testimony.
I agree with Mr. Badwan's answer. I don't see how this would make your boyfriend worse off. That is, unless the prosecution is somehow alleging that the change in story is due to some type of treat or coercion by your boyfriend or someone acting on his behalf. Otherwise, this goes directly to addressing the alleged victim's credibility, or lack thereof. It also gives the motive as to why she made up the story to begin with. In these type of sex cases, I feel if you can identify the motive as to why the alleged victim would lie, you are on your way to a not guilty verdict. You now have not only a motive for her lying, but her stating she in fact lied. It doesn't generally get much better than that without the government dismissing outright. Of course, there is no mention of any other potentially negative facts in your post.
Good luck to you and your boyfriend!
Generally, I would tend to believe he is better off . However, it depends on the wording of the statement and how it was obtained. If your friend had any influence on the girl, directly or indirectly, the statement could be used against him. He should have a candid conversation with his attorney.