Various groups use the phrase "a friend of so-and-so" as a discreet reference to or query about membership. "A friend of Bill (W.)" is a member of Alcoholics Anonymous, "a friend of Dorothy" is a gay guy, "a friend of Tom" is a Scientologist, etc.
I think atheists should have our own "friend". So... should an atheist be a friend of Charles? [Darwin] Richard? [Dawkins] David? [Hume] Paul? [Myers; "PZ" lacks euphony]
Sam and Chris don't make the short list because they're doofuses.
My preference is for "A friend of Richard."
Update: The Turkish Prawn adds the excellent suggestion: "A friend of Doug." [Adams] I have to say that's as appealing to me as "A friend of Richard."