This question allows for some gray area. Letter writers need to be objective, so letters from professional colleagues with close personal relationships with the applicant are discouraged. Specifically ...

Explain how you know them, how long you’ve known them, and in what capacity (e.g., coworker ... your friend in a professional setting while writing this letter of recommendation.

both on their resumes and in their day-to-day professional lives. Who knows? Their efforts may one day be rewarded by their colleagues with a glowing recommendation letter.