First, it depends on what the school's waitlist policy is. Make sure that they accept additional materials after you've been waitlisted (some, like Wharton, do NOT). Ignoring these instructions will only hurt yourchances.
If the school does accept additional materials, then your friend should only write one IF it will address something that you think is currently lacking in your app. If you're going to have him write a reco just because he's willing to, and you can't think of anything worth having him say, tehn it's not worth it.
Other than that, keep it to more than a paragraph or two. But most of all, make sure that you have a legitimate, focused reason for the letter.
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