Go to "My Friend Codes," and find the username that you're looking for. In the same bar, you should see a five white stars. Click on the first star to give a 1-star rating, the fifth star to give a 5-star rating, etc. You also can't leave a rating until you've added a comment about them on the same bar.
Hope that helped.
Signature-------------- Major Monogram: Sources tell us that Doofenshmirtz's "ROTFL". We don't know what that means, but it sounds dangerous.