I would tell her, because if I didn't, and she found out that I knew and said nothing... I might as well be guilty of sending him naked pictures. That's how my friends and I are, though. I would want to know. If my best friend came to me and said "Steph, Bill's been doing this, and here's the proof." - I would listen to my best friends. They'd never try to hurt me or lie to me, and would only give me the truth.
Especially if you have been one of the ones he has asked for naked pictures of, I would see it was your "duty" as her best friend to tell her. Take her out for coffee, lunch, dinner, whatever and just tell her what's going on.